Aqueous redox flow batteries comprising metal ligand coordination compounds

ABSTRACT

This invention is directed to aqueous redox flow batteries comprising redox-active metal ligand coordination compounds. The compounds and configurations described herein enable flow batteries with performance and cost parameters that represent a significant improvement over that previous known in the art.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/795,878, filed Mar. 12, 2013, which itself claims priorityto U.S. Application No. 61/739,145, filed Dec. 19, 2012, U.S.Application No. 61/738,546, filed Dec. 18, 2012, U.S. Application No.61/683,260, filed Aug. 15, 2012, and U.S. Application No. 61/676,473,filed Jul. 27, 2012. Each of the foregoing applications is incorporatedby reference in its entirety for any and all purposes.

TECHNICAL FIELD

This disclosure relates to the field of energy storage systems,including electrochemical energy storage systems, batteries, and flowbattery systems and methods of operating the same.

BACKGROUND

There exists a long-felt need for safe, inexpensive, easy-to-use, andreliable technologies for energy storage. Large-scale energy storageenables diversification of energy supply and optimization of the energygrid, including increased penetration and utilization of renewableenergies. Existing renewable-energy systems (e.g., solar- and wind-basedsystems) enjoy increasing prominence as energy producers explorenon-fossil fuel energy sources. However, storage is required to ensure areliable, high quality energy supply when sunlight is not available andwhen wind does not blow.

Electrochemical energy storage systems have been proposed forlarge-scale energy storage. To be effective, these systems must be safe,reliable, low-cost, and highly efficient at storing and producingelectrical power. Flow batteries, compared to other electrochemicalenergy storage devices, offer an advantage for large-scale energystorage applications owing to their unique ability to decouple thefunctions of power density and energy density. Existing flow batteries,however, have suffered from the reliance on battery chemistries thatresult in high costs of active materials and system engineering, lowcell and system performance (e.g. round trip energy efficiency), poorcycle life, and others.

Despite significant development effort, no flow battery technology hasyet achieved widespread commercial adoption. Accordingly, there is aneed in the art for improved flow battery chemistries and systems.

SUMMARY

The present invention addresses these challenges through the discoveryand implementation of a novel class of flow battery active materials.Traditional flow battery active materials comprise simple transitionmetal salts and/or halogen ions as positive|negative active materials inacidic or caustic electrolyte (e.g., iron-chrome: Fe^(3+/2+)|Cr^(3+/2+);vanadium: VO₂ ⁺/VO²⁺|V^(3+/2+); zinc-bromine: Zn(OH)₄ ²⁻/Zn|Br₂/Br⁻;hydrogen-bromine: H⁺/H₂|Br₂/Br⁻). In this configuration, the overallbattery properties (energy density, cell voltage, charge/discharge rate,etc) are limited by the inherent chemical properties of the basemetal/halogen ions. In particular, the negative couples taught by theprior art may each exhibit adequate electromotive force but with poorelectrode kinetics (e.g., Cr^(3+/2+)), exhibit modest electromotiveforce with modest electrode kinetics (e.g., V^(3+/2+)), plate metal ontothe negative electrode precluding the decoupling of stack size anddischarge time and presenting dendrite growth throughout cycling (e.g.,Zn^(2+/0)), or exhibit modest electromotive force and require themanagement of flammable gas (e.g., H⁺/H₂). Considerable attention hasbeen paid to overcoming these deficiencies, but to date, to littleavail.-Instead, the recent art in energy storage has largely taught newways of arranging and operating cell stacks and modifications toelectrolytes and electrodes that address minor deficiencies rather thanthe broad requirements of effectively storing energy.

This disclosure describes a novel class of compounds that unexpectedlyovercome the deficiencies presented by the prior art. The redox activemetal-ligand coordination compounds described herein provide activematerials comprising low-cost, earth abundant elements at useful batteryhalf-cell potentials. Unexpectedly, the materials were discovered toexhibit high solubility (allowing for high energy storage density) andhigh electromotive forces (e.g., including highly negative potentials)and suitably rapid electrode kinetics that enable operation of energystorage devices at high current densities. Through various choices ofcertain of these electrolyte, active material, and electrodecompositions, flow battery cells are enabled that operate at high cellvoltages and with high efficiency. Active materials that include acomposition of matter described by this invention may be used in energystorage systems in such a way that they are paired with other activematerials to form positive couples and negative couples wherein saidother active materials are described by the present invention or arepreviously known in the art or a combination thereof, including thosecomprising soluble, semi-solid, intercalation, capacitive orpseudo-capacitive, or plating-type active materials. That is, thepresent invention may be used in both half-cells of an energy storagesystem or as one half-cell in a system where the other half-cell is, forexample, Fe^(2+/3+), Br₂/Br⁻, H⁺/H₂, VO²⁺/VO₂ ⁺, or another half-cell.

Certain embodiments of the present invention provide flow batteries,each flow battery comprising a first aqueous electrolyte comprising afirst redox active material; a second aqueous electrolyte comprising asecond redox active material; a first electrode in contact with saidfirst aqueous electrolyte; a second electrode in contact with saidsecond aqueous electrolyte and a separator disposed between said firstaqueous electrolyte and said second aqueous electrolyte; wherein each ofthe first and second redox active materials comprise a metal ligandcoordination compound that independently exhibits substantiallyreversible electrochemical kinetics.

Other embodiments provide flow batteries, each flow battery comprising:a first aqueous electrolyte comprising a first redox active material; asecond aqueous electrolyte comprising a second redox active material; afirst electrode in contact with said first aqueous electrolyte; a secondelectrode in contact with said second aqueous electrolyte and aseparator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte; wherein the first or second redox activematerial, or both the first and second redox active materials comprise ametal ligand coordination compound having a formula comprisingM(L1)_(x)(L2)_(y)(L3)_(z) ^(m), where M is independently a non-zerovalent metal or metalloid of Groups 2-16, including lanthanides andactinides, where x, y, and z are independently 0, 1, 2, or 3 and1≦x+y+z≦3; m is independently −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, or 5;and L1, L2, and L3 are each independently ascorbate, citrate, aglycolate or polyol (including a ligand derived from ethylene glycol,propylene glycol, or glycerol), gluconate, glycinate, α-hydroxyalkanoate(e.g., α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, a phthalate, sarcosinate,salicylate, lactate, or a compound having structure according to FormulaI, or an oxidized or reduced form thereof:

wherein Ar is a 5-20 membered aromatic moiety, optionally comprising oneof more ring O, N, or S heteroatoms; X₁ and X₂ are independently —OH,—NHR′, —SH, or an anion thereof, X₁ and X₂ being positioned ortho to oneanother; R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆alkyl, C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, aboric acid or a salt thereof, carboxy acid or a salt thereof, C₂₋₆carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol (preferably polyethylene glycol, —[CH₂CH₂—O]₂₋₂₀—OH,preferably —[CH₂CH₂—O]₂₋₆—OH); R′ is independently H or C₁₋₃ alkyl; andn is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, preferably 0, 1, 2, 3, or 4.

Some embodiments provide certain separator characteristics, both inabsolute compositional and parametric terms and in relation to the metalligand coordination compounds. Other embodiments describe specificfunctional characteristics that derive from the inventive systems.

In still other embodiments, each flow battery comprises: a first aqueouselectrolyte comprising a first redox active material; a second aqueouselectrolyte comprising a second redox active material; a first carbonelectrode in contact with said first aqueous electrolyte; a secondcarbon electrode in contact with said second aqueous electrolyte and aseparator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte; wherein the first, second, or both first andsecond redox active material comprises a metal ligand coordinationcomplex having a formula comprising M(L1)_(x)(L2)_(y)(L3)_(z) Mcomprises Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, S, Sn, Ti, W, Zn, or Zr;L1, L2, and L3 are each independently ascorbate, a catecholate, citrate,a glycolate or polyol (including a ligand derived from ethylene glycol,propylene glycol, or glycerol), gluconate, glycinate, α-hydroxyalkanoate(e.g., α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, a phthalate, a pyrogallate,sarcosinate, salicylate, or lactate; where x, y, and z are independently0, 1, 2, or 3, and 1≦x+y+z≦3; and m is 5, −4, −3, −2, −1, 0, 1, 2, 3, 4,or 5 Related and independent embodiments provide that (a) x=3, y=z=0;(b) x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0;or (f) x=1, y=z=0. In certain preferred embodiments, M is Al, Cr, Fe,Mn, or Ti; and m is—+1, 0, −1, −2, −3, −4, or −5.

In still other independent embodiments, each flow battery comprises: afirst aqueous electrolyte comprising a first redox active material; asecond aqueous electrolyte comprising a second redox active material; afirst electrode in contact with said first aqueous electrolyte; a secondelectrode in contact with said second aqueous electrolyte and aseparator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte; wherein

-   -   (i) the first, second, or both redox active materials comprise a        metal ligand coordination compound in concentrations of at least        about 0.75 M; or    -   (ii) the flow battery is capable of operating with a current        density of at least about 100 mA/cm² and a round trip voltage        efficiency of at least about 70%; or    -   (iii) the separator has a thickness of 100 microns or less,        about 50 microns or less, or about 25 microns or less; or    -   (iv) the energy density of the electrolytes is at least 30 Wh/L;        or    -   (v) any combination of (i)-(iv).

The invention also provides systems, each system comprising a flowbattery as described herein, and further comprising:

(a) first chamber containing the first aqueous electrolyte and a secondchamber containing the second aqueous electrolyte;

(b) at least one electrolyte circulation loop in fluidic communicationwith each electrolyte chamber, said at least one electrolyte circulationloop comprising storage tanks and piping for containing and transportingthe electrolytes;

(c) control hardware and software; and

(d) an optional power conditioning unit.

Also, the invention provides methods of operating the flow batteries orsystems described herein, each method comprising charging said batteryby the input of electrical energy or discharging said battery by theremoval of electrical energy. In some of these embodiments, each method,with an associated flow of electrons, comprises applying a potentialdifference across the first and second electrode, so as to:

(a) reduce the first redox active metal ligand coordination compound; or

(b) oxidize the second redox active metal ligand coordination compound;or

(c) both (a) and (b).

In other embodiments, each method, with an associated flow of electrons,comprises applying a potential difference across the first and secondelectrode, so as to:

(a) oxidize the first redox active metal ligand coordination compound;or

(b) reduce the second redox active metal ligand coordination compound;or

(c) both (a) and (b).

BRIEF DESCRIPTION OF THE DRAWINGS

The present application is further understood when read in conjunctionwith the appended drawings. For the purpose of illustrating the subjectmatter, there are shown in the drawings exemplary embodiments of thesubject matter; however, the presently disclosed subject matter is notlimited to the specific methods, devices, and systems disclosed. Inaddition, the drawings are not necessarily drawn to scale. In thedrawings:

FIG. 1 depicts a schematic of an exemplary flow battery.

FIG. 2 provides stability performance data obtained during 250charge/discharge cycles for a 5 cm² system based on Ti^(4+/3+) (cat)₃^(2−/3−) and Fe^(3+/2+) (CN)₆ ^(3−/4−), as described in Example 2.

FIG. 3 provides a charge/discharge trace for a flow battery of thepresent invention as described in Example 2. This example containsTi^(4+/3+) (cat)₃ ^(2−/3−) and Fe^(3+/2+) (CN)₆ ^(3−/4−) as first andsecond electrolytes, respectively. The battery was charged from 0% SOCto 60% SOC and then discharged to 40% SOC at a current density of 200mA/cm² and a RT Voltage efficiency of ˜76%.

FIG. 4 provides current efficiency data obtained for a system based onTi^(4+/3+) (cat)₃ ^(2−/3−) and Fe^(3+/2+) (CN)₆ ^(3−/4−), as describedin Example 3.

FIG. 5 provides voltage efficiency data, as a function of currentdensity, for a system based on Ti^(4+/3+) (cat)₂(pyrogallate)^(2−/3−)and Fe^(3+/2+) (CN)₆ ^(3−/4−), as described in Example 4.

FIG. 6 provides voltage efficiency data, as a function of currentdensity, for a system based on Ti^(4+/3+) (cat)₃ ^(2−/3−) and Fe^(3+/2+)(CN)₆ ^(3−/4−), as described in Example 4.

FIG. 7 provides a charge/discharge trace for a flow battery of thepresent invention. This example contains Fe^(3+/2+) (cat)₃ ^(3−/4−)Fe^(3+/2+) (CN)₆ ^(3−/4−) as first and second electrolytes,respectively. The battery was charged from 0% SOC to 60% SOC and thendischarged to 40% SOC at a current density of 100 mA/cm² and a RTvoltage efficiency of ca. 82%.

FIG. 8 provides cyclic votammogram, CV traces for Al(cit)₂(cat)^(2−/3−)in pH 11.5 Na₂SO₄ electrolyte recorded at a glassy carbon electrode.

FIG. 9 provides CV traces for titanium tris-pyrogallate over a range ofoperating potentials. The data were generated using solutions of 75 mMNaK[Ti(pyrogallate)₃] at a pH of 9.8 and 1 M Na₂SO₄, recorded at aglassy carbon electrode.

FIG. 10 provides CV traces for iron tris-catecholate over a range ofoperating potentials. The data were generated using solutions of 1MNaK[Fe(catecholate)₃] at a pH of 11, and 3 M Na/KCl, recorded at aglassy carbon electrode.

FIG. 11 provides a CV trace for titanium bis-catecholatemono-pyrogallate over a range of operating potentials. The data weregenerated using solutions of 1.6 M NaK[Ti(catecholate)₂(pyrogallate)] ata pH of 11, recorded at a glassy carbon electrode.

FIG. 12 provides a CV trace for titanium bis-catecholate monolactateover a range of operating potentials. The data were generated usingsolutions of 0.75 M NaK[Ti(catecholate)₂(lactate)] at a pH of 9,recorded at a glassy carbon electrode.

FIG. 13 provides a CV trace for titanium bis-catecholate mono-gluconateover a range of operating potentials. The data were generated usingsolutions of 1.5 M NaK[Ti(catecholate)₂(gluconate)] at a pH of 9,recorded at a glassy carbon electrode.

FIG. 14 provides a CV trace for titanium bis-catecholate mono-ascorbateover a range of operating potentials. The data were generated usingsolutions of 1.5 M NaK[Ti(catecholate)₂(ascorbate)] at a pH of 10,recorded at a glassy carbon electrode.

FIG. 15 provides a CV trace for titanium tris-catecholate over a rangeof operating potentials. The data were generated using solutions of 1.5M Na₂-[Ti(catecholate)₃] at a pH of 11, recorded at a glassy carbonelectrode.

FIG. 16 provides a CV trace for titanium mono-catecholatemono-pyrogallate mono-lactate over a range of operating potentials. Thedata were generated using solutions of 1.5 MNaK[Ti(catecholate)(pyrogallate)(lactate)] at a pH of 8.5, recorded at aglassy carbon electrode.

FIG. 17 provides a CV trace for titanium tris-citrate over a range ofoperating potentials. The data were generated using solutions of 0.5 MNa₄-[Ti(citrate)₃] at a pH of 5, recorded at a platinum disk electrode.

FIG. 18 provides a CV trace from a solution of 1.5 M [Fe(CN)₆]⁴⁻obtained at a glassy carbon disk working electrode at several scan ratesusing 0.1 M sodium potassium hydrogen phosphate as the supportingelectrolyte, as described in Example 5.11. The ratio of Na⁺/K⁺counterions in this example was ca. 1:1.

FIG. 19 provides a CV trace for chromium hexacyanide over a range ofoperating potentials. The data were generated using solutions of 0.05 MK₃-[Cr(CN)₆] at a pH of 9, recorded at a glassy carbon electrode.

FIG. 20 provides a CV trace for manganese hexacyanide over a range ofoperating potentials. The data were generated using solutions of 0.1 MK₃[Mn(CN)₆] at a pH of 9, recorded at a glassy carbon electrode.

FIG. 21 provides data for cell voltage during charge-discharge cyclingfor 1 M Fe(CN)₆ as positive couple and 1 M Ti(lactate)₂(salicylate) asnegative couple, both at pH 11, in a 5 cm² active area flow battery at acurrent density of 150 mA/cm² except for the area noted as 100 mA/cm².

FIG. 22 provides cell voltage in volts plotted versus test time in hoursduring charge-discharge cycling and iV traces between each cycle for 1 MFe(CN)₆ as positive couple and 1 M Ti(lactate)₂(α-hydroxyacetate) asnegative couple, both at pH 11, in a 5 cm² active area flow battery at acurrent density of 150 mA/cm².

FIG. 23 provides CV traces for 10 mM titanium tris-salicylate at pH 8.6over a range of operating potentials at a glassy carbon electrode withNaKSO₄ supporting electrolyte.

FIG. 24 provides CV traces for 1 M iron tris-salicylate at pH 9.3 over arange of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte

FIG. 25 provides CV traces for 10 mM titanium mono-lactate at pH 5.6over a range of operating potentials, recorded at a glassy carbonelectrode with NaKSO₄ supporting electrolyte.

FIG. 26 provides CV traces for 1 M titanium mono-lactate at pH 9 over arange of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte.

FIG. 27 provides CV traces for 1 M titanium bis-lactate at pH 2 over arange of operating potentials, recorded at a glassy carbon electrodewith Na₂SO₄ supporting electrolyte.

FIG. 28 provides CV traces for 1 M titanium bis-lactate at pH 3.6 over arange of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte.

FIG. 29 provides CV traces for 0.75 M titanium bis-lactate at pH 9 overa range of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte.

FIG. 30 provides CV traces for 100 mM titanium-bis-malate-mono-lactateat pH 9.9 over a range of operating potentials, recorded at a glassycarbon electrode with NaKSO₄ supporting electrolyte.

FIG. 31 provides CV traces for 200 mMtitanium-bis-malate-mono-salicylate at pH 10 over a range of operatingpotentials, recorded at a glassy carbon electrode with NaKSO₄ supportingelectrolyte.

FIG. 32 provides CV traces for 0.5 M titanium bis-lactate mono-glycinateat pH 9.9 over a range of operating potentials, recorded at a glassycarbon electrode with NaKSO₄ supporting electrolyte.

FIG. 33 provides CV traces for 0.5 M titanium bis-lactatemono-salicylate at pH 10 over a range of operating potentials, recordedat a pH of 9.3 at a glassy carbon electrode with NaKSO₄ supportingelectrolyte.

FIG. 34 provides CV traces for 0.5 M titanium bis-salicylatemono-lactate at pH 9.8 over a range of operating potentials, recorded ata glassy carbon electrode with NaKSO₄ supporting electrolyte.

FIG. 35 provides CV traces for 200 mM titanium bis-(α-hydroxyacetate)mono-salicylate over a range of operating potentials, recorded at a pHof 10 at a glassy carbon electrode with NaKSO₄ supporting electrolyte.

FIG. 36 provides CV traces for 0.5 M titanium bis-(α-hydroxyacetate)mono-lactate at pH 10 over a range of operating potentials at a glassycarbon electrode with NaKSO₄ supporting electrolyte.

FIG. 37 provides CV traces for 1 M iron tris-malate at pH 9.2 over arange of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte.

FIG. 38 provides CV traces for 1.5 M iron tris-(α-hydroxyacetate) at pH8.1 over a range of operating potentials, recorded at a glassy carbonelectrode with NaKSO₄ supporting electrolyte.

FIG. 39 provides CV traces for 1 M iron mono-lactate at pH 3.1 over arange of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte.

FIG. 40 provides CV traces for 1.5 M iron bis-lactate at pH 2.6 over arange of operating potentials, recorded at a glassy carbon electrodewith NaKSO₄ supporting electrolyte.

FIG. 41 provides CV traces for 1 M iron mono-lactate bis-glycinate at pH6.7 over a range of operating potentials, recorded at a glassy carbonelectrode with NaKSO₄ supporting electrolyte.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present disclosure may be understood more readily by reference tothe following description taken in connection with the accompanyingFigures and Examples, all of which form a part of this disclosure. It isto be understood that this disclosure is not limited to the specificproducts, methods, conditions or parameters described and/or shownherein, and that the terminology used herein is for the purpose ofdescribing particular embodiments by way of example only and is notintended to be limiting of any claimed disclosure. Similarly, unlessspecifically otherwise stated, any description as to a possiblemechanism or mode of action or reason for improvement is meant to beillustrative only, and the invention herein is not to be constrained bythe correctness or incorrectness of any such suggested mechanism or modeof action or reason for improvement. Throughout this text, it isrecognized that the descriptions refer both to methods of operating adevice and systems and to the devices and systems providing saidmethods. That is, where the disclosure describes and/or claims a methodor methods for operating a flow battery, it is appreciated that thesedescriptions and/or claims also describe and/or claim the devices,equipment, or systems for accomplishing these methods.

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art, and so forth.

When a value is expressed as an approximation by use of the descriptor“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function. The person skilledin the art will be able to interpret this as a matter of routine. Insome cases, the number of significant figures used for a particularvalue may be one non-limiting method of determining the extent of theword “about.” In other cases, the gradations used in a series of valuesmay be used to determine the intended range available to the term“about” for each value. Where present, all ranges are inclusive andcombinable. That is, references to values stated in ranges include everyvalue within that range.

When a list is presented, unless stated otherwise, it is to beunderstood that each individual element of that list and everycombination of that list is to be interpreted as a separate embodiment.For example, a list of embodiments presented as “A, B, or C” is to beinterpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A orC,” “B or C,” or “A, B, or C.”

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.That is, unless obviously incompatible or specifically excluded, eachindividual embodiment is deemed to be combinable with any otherembodiment(s) and such a combination is considered to be anotherembodiment. Conversely, various features of the invention that are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any sub-combination. Further, while anembodiment may be described as part of a series of steps or part of amore general structure, each said step or part may also be considered anindependent embodiment in itself. Additionally, while the chemistriesdescribed in the present disclosure are described in terms of flowbatteries, it should be appreciated that each of the chemical structuresor compositions described or exemplified herein, either by themselves oras electrolytes, are considered independent embodiments of the presentinvention (including the specific mixed ligand genera and structuresdescribed in terms of M(L1)_(x)(L2)_(y)(L3)_(z) ^(m) as describedbelow).

Electrochemical energy storage systems typically operate through theinterconversion of electrical and chemical energy. Various embodimentsof electrochemical energy storage systems include batteries, capacitors,reversible fuel cells and the like, and the present invention maycomprise any one or combination of these systems.

Unlike typical battery technologies (e.g., Li-ion, Ni-metal hydride,lead-acid, etc.), where energy storage materials and membrane/currentcollector energy conversion elements are unitized in a single assembly,flow batteries transport (e.g., via pumping) redox active energy storagematerials from storage tanks through an electrochemical stack, as inexemplary FIG. 1, which is described elsewhere herein in further detail.This design feature decouples the electrical energy storage system power(kW) from the energy storage capacity (kWh), allowing for considerabledesign flexibility and cost optimization.

In some embodiments, flow batteries according to the present disclosuremay also be described in terms of a first chamber comprising a first ornegative electrode contacting a first aqueous electrolyte; a secondchamber comprising a second or positive electrode contacting a secondaqueous electrolyte; and a separator disposed between the first andsecond electrolytes. The electrolyte chambers provide separatereservoirs within the cell, through which the first and/or secondelectrolyte flow so as to contact the respective electrodes and theseparator. Each chamber and its associated electrode and electrolytedefines its corresponding half-cell. The separator provides severalfunctions which include, e.g., (1) serving as a barrier to mixing offirst and second electrolytes; (2) electronically insulating to reduceor prevent short circuits between the positive and negative electrodes;and (3) to provide for ion transport between the positive and negativeelectrolyte chambers, thereby balancing electron transport during chargeand discharge cycles. The negative and positive electrodes provide asurface for electrochemical reactions during charge and discharge.During a charge or discharge cycle, electrolytes may be transported fromseparate storage tanks through the corresponding electrolyte chambers.In a charging cycle, electrical power is applied to the system whereinthe active material contained in the second electrolyte undergoes aone-or-more electron oxidation and the active material in the firstelectrolyte undergoes a one-or-more electron reduction. Similarly, in adischarge cycle the second electrolyte is reduced and the firstelectrolyte is oxidized producing electrical power.

Certain embodiments of the current invention provide flow batteries,each flow battery comprising:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein each of the first and second redox active materials comprise ametal ligand coordination compound that independently exhibitssubstantially reversible electrochemical kinetics. Either or both of theelectrodes that carry out the electrochemical reactions may comprisecarbon. The term “metal ligand coordination compound is described below,both in broad and more specific terms and each comprises separateembodiments.

As used herein, unless otherwise specified, the term “substantiallyreversible electrochemical kinetics” refers to the condition wherein thevoltage difference between the anodic and cathodic peaks is less thanabout 0.3 V, as measured by cyclic voltammetry, using an ex-situapparatus comprising a flat glassy carbon disc electrode and recordingat 100 mV/s. However, additional embodiments provide that the voltagedifference between the anodic and cathodic peaks is less than about 0.2V, less than about 0.1 V, less than about 0.075 V, or less than about0.059 V, under these same testing conditions.

Certain aspects of the invention also provide flow batteries, each ofwhich comprises:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the first or second redox active material, or both the first andsecond redox active materials comprise a metal ligand coordinationcompound having a formula comprising M(L1)_(x)(L2)_(y)(L3)_(z) ^(m),where M is independently a non-zero valent metal or metalloid of Groups2-16, including the lanthanide and actinide elements;

where x, y, and z are independently 0, 1, 2, or 3, and 1≦x+y+z≦3;

m is independently −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, or 5; and

L1, L2, and L3 are each independently ascorbate, citrate, a glycolate orpolyol (including a ligand derived from ethylene glycol, propyleneglycol, or glycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g.,α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, a phthalate, sarcosinate,salicylate, lactate, or a compound having structure according to FormulaI, or an oxidized or reduced form thereof:

wherein

Ar is a 5-20 membered aromatic moiety, optionally comprising one of moreO, N, or S heteroatoms;

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof, X₁ andX₂ being positioned ortho to one another (or otherwise positioned onadjacent carbon atoms on the aromatic or heteroaromatic ring system);

R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆ alkyl, C₁₋₆alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, a boric acid ora salt thereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano,halo, hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol(preferably polyethylene glycol, —[CH₂CH₂—O]₂₋₂₀—OH, preferably—[CH₂CH₂—O]₂₋₆—OH);

R′ is independently H or C₁₋₃ alkyl; and

n is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, preferably 0, 1, 2, 3, or 4.

Either or both of these electrodes may comprise carbon and either orboth of the first and second metal ligand coordination compoundindependently exhibits substantially reversible electrochemicalkinetics. Similarly, in either case, separate embodiments provide that(a) x=3, y=z=0; (b) x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0;(e) x=2, y=z=0; or (f) x=1, y=z=0. Throughout this disclosure, thephrase “a salts thereof” includes base salts such as, those formed withcations such as sodium, potassium, lithium, calcium, magnesium, ammoniumand alkylammonium. Other salts that do not negatively impact theoperation of the cell can also be utilized.

In those embodiments where the first and second aqueous electrolyteseach comprises a first and second metal ligand coordination compound,respectively, the first and second metal ligand coordination compoundsmay either be the same or different.

The invention also provides those embodiments were either the first orthe second or both the first and second metal ligand coordinationcompound comprises at least one ligand having a structure according toFormula I. Similarly, either or both of the metal ligand coordinationcompounds may comprise at least one ligand having a structure accordingto Formula IA, IB, or IC:

wherein

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof;

R₁ is independently H, C₁₋₆ alkoxy, C₁₋₆ alkyl, a boric acid or a saltthereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano, halo,hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol(preferably polyethylene glycol, —[CH₂CH₂—O]₂₋₂₀—OH, preferably—[CH₂CH₂—O]₂₋₆—OH);

R′ is independently H or C₁₋₃ alkyl; and

n is 0, 1, 2, 3, or 4.

Additional embodiments provide either or both of the metal ligandcoordination compounds comprises at least one ligand having a structureaccording to Formula IA, IB, or IC, but where:

X₁ and X₂ are both OH or an anion thereof;

R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid or a saltthereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano, halo,hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol(preferably polyethylene glycol, —[CH₂CH₂—O]₂₋₁₀—OH, more preferably—[CH₂CH₂—O]₂₋₆—OH); and

n is 1.

In various embodiments, either each or both of the first or second metalligand coordination compound may also comprise at least one ascorbate,catecholate, citrate, glycolate or polyol (including a ligand derivedfrom ethylene glycol, propylene glycol, or glycerol), gluconate,glycinate, α-hydroxyalkanoate (e.g., α-hydroxyacetate, from glycolicacid), β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate,phthalate, pyrogallate, sarcosinate, salicylate, or lactate ligand.

While the metal coordination compounds described in the broadest contextabove have been described in terms of non-zero valent metal or metalloidof Groups 2-16, including the lanthanide and actinide elements,additional embodiments provide that these metals may include non-zerovalent Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, Si, Sn, Ti, W, Zn, or Zr, forexample Al³⁺, Ca²⁺, Ce⁴⁺, Co³⁺, Cr³⁺, Fe³⁺, Mg²⁺, Mn³⁺, Mo⁶⁺, Si⁴⁺,Sn⁴⁺, Ti⁴⁺, W⁶⁺, Zn²⁺, or Zr⁴⁺. As described above, the first and secondmetal coordination compound may comprise the same or different non-zerovalent metal or metalloid, or the same element having a redox couple ofdiffering oxidation states. Metal ligand coordination compoundscomprising Al, Cr, Ti, Mn, or Fe are preferred, in either or both of thefirst or second compounds. In certain embodiments, the second metalligand coordination compound comprises an iron hexacyanide compound, forexample as a ferro-/ferricyanide couple.

As is discussed further below, the present invention also provides thateither or both of the first or the second metal ligand coordinationcompound are present in the first or second electrolyte, respectively,at elevated concentrations, for example at least about 0.5 M, at leastabout 0.6 M, at least about 0.75 M, or at least about 1 M. Higherconcentrations are preferred for yielding higher system energydensities. In separate independent embodiments, the energy density ofthe electrolytes is at least about 10 Wh/L, at least about 20 Wh/L, orat least about 30 Wh/L.

It is also important to reiterate that the individual embodimentsdescribed herein also include those where either or both, preferablyboth, of the first and second metal ligand coordination compounds eachexhibit substantially reversible electrochemical kinetics. Similarly,the flow batteries described herein, whether or not dependent on thespecific metal ligand combinations described are capable of providing(and do provide when operating) high round trip voltage and currentefficiencies, each of at least 70%, when measured at 200 mA/cm² and suchperformance features are considered individual embodiments of thepresent invention. Similarly, the present invention provides flowbatteries capable of operating, or operating, with a current density ofat least about 100 mA/cm² and a round trip voltage efficiency of atleast about 70%, at least about 80%, or at least about 90%. Thesefeatures can be realized even when the separator has a thickness ofabout 100 micron or less, about 50 micron or less, or about 25 micronsor less.

In other independent embodiments, flow batteries each comprise:

a first aqueous electrolyte comprising a metal ligand coordinationcompound;

a second aqueous electrolyte comprising a second metal ligandcoordination compound;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein at least one metal ligand coordination compound has a formulacomprising M(L1)_(x)(L2)_(y)(L3)_(z) where x, y, and z are independently0, 1, 2, or 3 and 1≦x+y+z≦3;

m is −5, −4, −3, −2, −1, 0, +1, +2, +3, +4, or +5;

M is Al, Cr, Fe, Mn, or Ti;

and L1, L2, and L3 are each independently ascorbate, a catecholate,citrate, a glycolate or polyol (including a ligand derived from ethyleneglycol, propylene glycol, or glycerol), gluconate, glycinate,α-hydroxyalkanoate (e.g., α-hydroxyacetate, from glycolic acid),β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, a phthalate, apyrogallate, sarcosinate, salicylate, or lactate. Related andindependent embodiments provide that (a) x=3, y=z=0; (b) x=2, y=1, z=0;(c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0; or (f) x=1, y=z=0.The terms “a catecholate” and “a pyrogallolate” reflect the fact thatthese ligands may be optionally substituted with at least one R₁ group,as defined above—i.e., in independent embodiments, the catecholate orpyrogallate are substituted and unsubstituted The catechol- orpyrogallol-type ligands may also be optionally substituted with C₁₋₆alkoxy (e.g., —O—C₁₋₆ alkyl), C₁₋₆ alkyl, C₁₋₆ alkenyl, C₁₋₆ alkynyl,5-6 membered aryl or heteroaryl, a boric acid or a salt thereof, acarboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano, halo, hydroxyl,nitro, sulfonate, sulfonic acid or a salt thereof, phosphonate,phosphonic acid or a salt thereof, or a polyglycol (preferablypolyethylene glycol, —[CH₂CH₂—O]₂₋₂₀—OH, preferably —[CH₂CH₂—O]₂₋₆—OH).

Throughout this disclosure, where L1, L2, and L3 are said to beindependently ascorbate, a catecholate, citrate, a glycolate or polyol(including ligands derived from ethylene glycol, propylene glycol, orglycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g.,α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, sarcosinate, salicylate,or lactate, it should be appreciated that these ligands are reflectiveof a broader class of ligands—i.e., those including aliphaticpolyhydroxy, carboxylic, polycarboxylic, or mixed hydroxy-carboxylicspecies capable of binding, preferably as bi-, tri-, or polydentatechelants, including C₂-C₁₀ α-, β-, and γ-hydroxy- orpolyhydroxycarboxylic acids (e.g., glycolic acid, sugars such asfructose, glucose) or C₃-C₁₀ hydroxy- or polyhydroxydi-, tri-, orpoly-carboxylic acids such as EDTA or DTPA. Coordination compoundscontaining this broader class of ligands are also considered within thescope of the present invention, especially when those coordinationcompounds also contain at least one catecholate or pyrogallate ligand,at least one polyhydroxy or polycarboxylate, α- and/orβ-hydroxycarboxylic acid ligand.

In certain independent embodiments, flow batteries each comprise:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the second redox active material comprises a metal ligandcoordination complex having a formula comprisingM(L1)_(x)(L2)_(y)(L3)_(z) ^(m),

M is Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, S, Sn, Ti, W, Zn, or Zr;

L1, L2, and L3 are each each independently ascorbate, a catecholate,citrate, a glycolate or polyol (including ligands derived from ethyleneglycol, propylene glycol, or glycerol), gluconate, glycinate,α-hydroxyalkanoate (e.g., α-hydroxyacetate, from glycolic acid),β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, a phthalate, apyrogallate, sarcosinate, salicylate, or lactate;

x, y, and z are independently 0, 1, 2, or 3, and 1≦x+y+z≦3;

and m is +1, 0, −1, −2, −3, −4, or −5. Related and independentembodiments provide that (a) x=3, y=z=0; (b) x=2, y=1, z=0; (c) x=1,y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0; or (f) x=1, y=z=0. Inpreferred embodiments, M is Al, Cr, Fe, or Ti.

In other embodiments, flow batteries each comprise:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the first, second, or both redox active materials comprise ametal ligand coordination compound in concentrations of at least about0.5 M, at least about 0.75 M, or at least about 1 M.

In other independent embodiments, flow batteries each comprise:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the first and second redox active materials comprise metalligand coordination compounds and said flow battery is capable ofoperating with a current density of at least about 100 mA/cm² and around trip voltage efficiency of at least about 70%.

In other independent embodiments, flow batteries each comprise:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the first and second redox active materials comprise metalligand coordination compounds; and

wherein the separator has a thickness of about 100 microns or less. Theseparator may also have a thickness of about 50 micron or less or about25 micron or less.

In other independent embodiments, flow batteries each comprise

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the first, second, or both redox active materials comprise metalligand coordination compounds; and

wherein the energy density of the electrolytes is at least about 20 Wh/Lor at least about 30 Wh/L.

Again, each of the specific features described above may be combinablesuch that the combined features form additional embodiments.

Additionally, in some embodiments, the flow batteries include thosewhere either the first or the second or both the first and second metalligand coordination compound comprises at least one ligand having astructure according to Formula I. In other embodiments, either one orboth of the metal ligand coordination compounds comprises at least oneligand having a structure according to Formula IA, IB, or IC:

wherein

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof;

R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid or a saltthereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano, halo,hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol(preferably polyethylene glycol, —[CH₂CH₂—O]₂₋₁₀—OH, more preferably—[CH₂CH₂—O]₂₋₆—OH);

R′ is independently H or C₁₋₃ alkyl; and

n is 0, 1, 2, 3, or 4.

In other embodiments, the either one or both of the metal ligandcoordination compounds comprises at least one ligand having a structureaccording to Formula IA, IB, or IC, wherein X₁ and X₂ are both OH or ananion thereof; R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boricacid or a salt thereof, carboxy acid or a salt thereof, C₂₋₆carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol (preferably polyethylene glycol, —[CH₂CH₂—O]₂₋₁₀—OH, morepreferably —[CH₂CH₂—O]₂₋₆—OH); and n is 1.

The invention also contemplates those embodiments wherein the at leastone metal ligand coordination compound comprises (a) at least onecatecholate or pyrogallate ligand, (b) at least one ascorbate, citrate,a glycolate or polyol (including ligands derived from ethylene glycol,propylene glycol, or glycerol), gluconate, glycinate, α-hydroxyalkanoate(e.g., α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, sarcosinate, salicylate,or lactate ligand, or (c) both at least one catecholate or pyrogallateligand, and at least one ascorbate, citrate, a glycolate or polyol(including ligands derived from ethylene glycol, propylene glycol, orglycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g.,α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, sarcosinate, salicylate,or lactate ligand. Similarly, the second metal ligand coordinationcompound may comprise (a) at least one catecholate or pyrogallateligand, (b) at least one ascorbate, citrate, a glycolate or polyol(including a ligand derived from ethylene glycol, propylene glycol, orglycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g.,α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, sarcosinate, salicylate,or lactate ligand, or (c) both at least one catecholate or pyrogallateligand, and at least one ascorbate, citrate, a glycolate or polyol(including a ligand derived from ethylene glycol, propylene glycol, orglycerol), gluconate, glycinate, α-hydroxyalkanoate (e.g.,α-hydroxyacetate, from glycolic acid), β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, sarcosinate, salicylate,or lactate ligand. In some of these embodiments, at least one metalligand coordination compound is a chromium, iron, manganese, molybdenum,or ruthenium cyanide compound, preferably a chromium, iron, or manganesehexacyanide, such as ferricyanide or ferrocyanide in combination anothermetal ligand coordination compound as described herein.

The invention further contemplates those embodiments within the scope ofGroups A-F, wherein the first and second metal ligand coordinationcompounds each exhibit substantially reversible electrochemicalkinetics.

To this point, the various embodiments have been described mainly interms of individual flow batteries. It should be appreciated that, wherepossible, the descriptions should be read as including flow batteriesthat are operating or capable of operating with the specifiedcharacteristics. Similarly, the descriptions should be read as includingsystems of flow batteries,

wherein the system comprises at least two of the flow batteriesdescribed herein.

An exemplary flow battery is shown in FIG. 1. As shown in that figure, aflow battery system may include an electrochemical cell that features aseparator 20 (e.g., a membrane) that separates the two electrodes of theelectrochemical cell. Electrode 10 is suitably a conductive material,such as a metal, carbon, graphite, and the like. Tank 50 may containfirst redox material 30, which material is capable of being cycledbetween an oxidized and reduced state.

A pump 60 may affect transport of the first active material 30 from thetank 50 to the electrochemical cell. The flow battery also suitablyincludes a second tank (not labeled) that contains the second activematerial 40. The second active material 40 may or may not be the same asactive material 30. A second pump (not labeled) may affect transport ofsecond redox material 40 to the electrochemical cell. Pumps may also beused to affect transport of the active materials from theelectrochemical cell to the tanks of the system. Other methods ofeffecting fluid transport—e.g., siphons—may be used to transport redoxmaterial into and out of the electrochemical cell. Also shown is a powersource or load 70, which completes the circuit of the electrochemicalcell and allows the user to collect or store electricity duringoperation of the cell.

It should be understood that FIG. 1 depicts a specific, non-limitingembodiment of a flow battery. Accordingly, devices according to thepresent disclosure may or may not include all of the aspects of thesystem depicted in FIG. 1. As one example, a system according to thepresent disclosure may include active materials that are solid, liquid,or gas and/or solids, liquids, or gases dissolved in solution orslurries. Active materials may be stored in a tank, in a vessel open tothe atmosphere, or simply vented to the atmosphere.

In some cases, a user may desire to provide higher charge or dischargevoltages than available from a single battery. In such cases, and incertain embodiments, then, several batteries are connected in seriessuch that the voltage of each cell is additive. An electricallyconductive, but non-porous material (e.g., a bipolar plate) may beemployed to connect adjacent battery cells in a bipolar stack, whichallows for electron transport but prevents fluid or gas transportbetween adjacent cells. The positive electrode compartments and negativeelectrode compartments of individual cells are suitably fluidicallyconnected via common positive and negative fluid manifolds in the stack.In this way, individual electrochemical cells can be stacked in seriesto yield a desired operational voltage.

In additional embodiments, the cells, cell stacks, or batteries areincorporated into larger energy storage systems, suitably includingpiping and controls useful for operation of these large units. Piping,control, and other equipment suitable for such systems are known in theart, and include, for example, piping and pumps in fluid communicationwith the respective electrochemical reaction chambers for movingelectrolytes into and out of the respective chambers and storage tanksfor holding charged and discharged electrolytes. The energy storage andgeneration systems described by the present disclosure may also includeelectrolyte circulation loops, which may comprise one or more valves,one or more pumps, and optionally a pressure equalizing line. The energystorage and generation systems of this disclosure can also include anoperation management system. The operation management system may be anysuitable controller device, such as a computer or microprocessor, andmay contain logic circuitry that sets operation of any of the variousvalves, pumps, circulation loops, and the like.

In some embodiments, a flow battery system may comprise a flow battery(including a cell or cell stack), a first chamber containing the firstaqueous electrolyte and a second chamber containing the second aqueouselectrolyte; at least one electrolyte circulation loop in fluidiccommunication each electrolyte chamber, said at least one electrolytecirculation loop comprising storage tanks and piping for containing andtransporting the electrolytes; control hardware and software (which mayinclude safety systems); and an optional power conditioning unit. Theflow battery cell stack accomplishes the conversion of charging anddischarging cycles and determines the peak power of energy storagesystem, which power may in some embodiments be in the kW range. Thestorage tanks contain the positive and negative active materials; thetank volume determines the quantity of energy stored in the system,which may be measured in kWh. The control software, hardware, andoptional safety systems suitably include sensors, mitigation equipmentand other electronic/hardware controls and safeguards to ensure safe,autonomous, and efficient operation of the flow battery energy storagesystem. Such systems are known to those of ordinary skill in the art. Apower conditioning unit may be used at the front end of the energystorage system to convert incoming and outgoing power to a voltage andcurrent that is optimal for the energy storage system or theapplication. For the example of an energy storage system connected to anelectrical grid, in a charging cycle the power conditioning unit wouldconvert incoming AC electricity into DC electricity at an appropriatevoltage and current for the electrochemical stack. In a dischargingcycle, the stack produces DC electrical power and the power conditioningunit converts to AC electrical power at the appropriate voltage andfrequency for grid applications.

The energy storage systems of the present disclosure are, in someembodiments, suited to sustained charge or discharge cycles of severalhour durations. For example, in some embodiments, the flow batteries ofthe present invention are capable of retaining at least about 70%efficiency when subjected to 10 charge/discharge cycles. As such, thesystems of the present disclosure may be used to smooth energysupply/demand profiles and provide a mechanism for stabilizingintermittent power generation assets (e.g., from renewable energysources). It should be appreciated, then, that various embodiments ofthe present disclosure include those electrical energy storageapplications where such long charge or discharge durations are valuable.For example, non-limiting examples of such applications include thosewhere systems of the present disclosure are connected to an electricalgrid include, so as to allow renewables integration, peak load shifting,grid firming, baseload power generation consumption, energy arbitrage,transmission and distribution asset deferral, weak grid support, and/orfrequency regulation. Cells, stacks, or systems according to the presentdisclosure may be used to provide stable power for applications that arenot connected to a grid, or a micro-grid, for example as power sourcesfor remote camps, forward operating bases, off-grid telecommunications,or remote sensors.

Flow battery energy storage efficacy is determined by both the roundtrip DC-DC energy efficiency (RT_(EFF)) and the energy density of theactive materials (measured in Wh/L). The RT_(EFF) is a composite ofvoltage and current efficiencies for both the battery charge anddischarge cycles. In electrochemical devices, voltage and currentefficiencies are functions of the current density, and while voltage andcurrent efficiency typically decrease as current density (mA/cm²)increases, high current densities are often desirable to reduceelectrochemical stack size/cost required to achieve a given powerrating. Active material energy density is directly proportional to thecell OCV (OCV=open circuit voltage), the concentration of activespecies, and the number of electrons transferred per mole of activespecies. High energy densities are desirable to reduce the volume ofactive materials required for a given quantity of stored energy.

It should be appreciated that, while the various embodiments describedherein are described in terms of flow battery systems, the samestrategies and design/chemical embodiments may also be employed withstationary (non-flow) electrochemical cells, batteries, or systems,including those where one or both half cells employ stationaryelectrolytes. Each of these embodiments is considered within the scopeof the present invention.

Terms

Throughout this specification, words are to be afforded their normalmeaning, as would be understood by those skilled in the relevant art.However, so as to avoid misunderstanding, the meanings of certain termswill be specifically defined or clarified.

The term “active material” is well known to those skilled in the art ofelectrochemistry and electrochemical energy storage and is meant torefer to materials which undergo a change in oxidation state duringoperation of the system. Active materials may comprise a solid, liquid,or gas and/or solids, liquids, or gasses dissolved in solution. Incertain embodiments, active materials comprise molecules and/orsupramolecules dissolved in solution. Active materials with acomposition of matter described by this invention may be used in energystorage systems in such a way that they are paired with other activematerials to form a positive couple and a negative couple wherein saidother active materials are described by the present invention or arepreviously known in the art or a combination thereof, inclusive ofsoluble, semi-solid, intercalation, capacitive or pseudo-capacitive, andplating-type active materials. The concentration of the molecules may beat least about 2 M, between about 1 M and about 2 M, about 1.5 M,between 0.5 M and 1M, or 0.5 M or less.

In certain embodiments, the active material may comprise a “metal ligandcoordination compound,” which are known to those skilled in the art ofelectrochemistry and inorganic chemistry. A metal ligand coordinationcompound may comprise a metal ion bonded to an atom or molecule. Thebonded atom or molecule is referred to as a “ligand”. In certainnon-limiting embodiments, the ligand may comprise a molecule comprisingC, H, N, and/or O atoms. In other words, the ligand may comprise anorganic molecule. The metal ligand coordination compounds of the presentdisclosure are understood to comprise at least one ligand that is notwater, hydroxide, or a halide (F⁻, Cr⁻, Br⁻, I⁻). Where presented hereas being represented by “M(L1)_(x)(L2)_(y)(L3)_(z) ^(m), x, y, and z areindependently 0, 1, 2, or 3, such that 1≦x+y+z≦3” it should beappreciated that this reflects independent embodiments where “M”contains 1, 2, or 3 ligands of L1, L2, and L3 within its innercoordination sphere, where L1, L2, and L3 are different from oneanother.

Metal ligand coordination compounds may comprise a “redox active metalion” and/or a “redox inert metal ion”. The term “redox active metal ion”is intended to connote that the metal undergoes a change in oxidationstate under the conditions of use. As used herein, the term “redoxinert” metal ion is intended to connote that the metal does not undergoa change in oxidation state under the conditions of use. Metal ions maycomprise non-zero valence salts of, e.g., Al, Ca, Co, Cr, Sr, Cu, Fe,Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn, Ti, Zn, Zr, V, or a combination thereof.The skilled artisan would be able to recognize the circumstances where agiven non-zero valence metal would be redox active or inactive under theprescribed electrolyte environments.

In other embodiments, the active material may comprise an “organicactive material”. An organic active material may comprise a molecule orsupramolecule that does not contain a transition metal ion. It isfurther understood that organic active materials are meant to comprisemolecules or supramolecules that are dissolved in aqueous solution. Andorganic active material is capable of undergoing a change in oxidationstate during operation of the electrochemical energy storage system. Inthis case, the molecule or supramolecule may accept or donate anelectron during operation of the system.

Unless otherwise specified, the term “aqueous” refers to a solventsystem comprising at least about 98% by weight of water, relative tototal weight of the solvent. In some applications, soluble, miscible, orpartially miscible (emulsified with surfactants or otherwise)co-solvents may also be usefully present which, for example, extend therange of water's liquidity (e.g., alcohols/glycols). When specified,additional independent embodiments include those where the “aqueous”solvent system comprises at least about 55%, at least about 60 wt %, atleast about 70 wt %, at least about 75 wt %, at least about 80%, atleast about 85 wt %, at least about 90 wt %, at least about 95 wt %, orat least about 98 wt % water, relative to the total solvent. It somesituations, the aqueous solvent may consist essentially of water, and besubstantially free or entirely free of co-solvents or other species. Thesolvent system may be at least about 90 wt %, at least about 95 wt %, orat least about 98 wt % water, and, in some embodiments, be free ofco-solvents or other species.

In addition to the redox active materials described below, the aqueouselectrolytes may contain additional buffering agents, supportingelectrolytes, viscosity modifiers, wetting agents, and the like.

The term “bipolar plate” refers to an electrically conductive,substantially nonporous material that may serve to separateelectrochemical cells in a cell stack such that the cells are connectedin series and the cell voltage is additive across the cell stack. Thebipolar plate has two surfaces such that one surface of the bipolarplate serves as a substrate for the positive electrode in one cell andthe negative electrode in an adjacent cell. The bipolar plate typicallycomprises carbon and carbon containing composite materials.

The term “cell potential” is readily understood by those skilled in theart of electrochemistry and is defined to be the voltage of theelectrochemical cell during operation. The cell potential may be furtherdefined by Equation 1:

Cell Potential=OCV−η_(pos)−η_(neg) −iR  (1)

where OCV is the “open circuit potential”, η_(pos) and η_(neg) are theoverpotentials for the positive and negative electrodes at a givencurrent density, respectively, and iR is the voltage loss associatedwith all cell resistances combined. The “open circuit potential” or OCVmay be readily understood according to Equation 2:

OCV═E ⁺ −E ⁻  (2)

where E⁺ and E⁻ are the “half-cell potentials” for the redox reactionstaking place at the positive and negative electrodes, respectively. Thehalf-cell potentials may be further described by the well-known NernstEquation 3:

E=E°−RT/nF ln(X _(red) /X _(ox))  (3)

wherein E° is the standard reduction potential for redox couple ofinterest (e.g., either the positive or negative electrode), the R is theuniversal gas constant, T is temperature, n is the number of electronstransferred in the redox couple of interest, F is Faraday's constant,and X_(red)/X_(ox) is the ratio of reduced to oxidized species at theelectrode.

The OCV of a battery system may be measured by using standard techniqueswhen the current flow between the first and second electrode is equal tozero. In this condition, the voltage difference between the first andsecond electrodes corresponds to the OCV. The OCV of a battery systemdepends on the state of charge (SOC) of said system. Without being boundto the correctness of any theory, the OCV of an ideal battery, willchange with state of charge according to the Nernst equation (equation 4above). For simplicity in this application all OCVs will be referencedto their values at 50% SOC. Those of ordinary skill in the art willrecognize that at higher SOCs the OCV of a battery will increase, and atlower SOCs the OCV will decrease from the value at 50% SOC.

The term “current density” refers to the total current passed in anelectrochemical cell divided by the geometric area of the electrodes ofthe cell and is commonly reported in units of mA/cm². In certainembodiments of the present invention, current densities may be in arange of from about 50 mA/cm², from about 100 mA/cm² or from about 200mA/cm², to about 200 mA/cm², to about 300 mA/cm², to about 400 mA/cm²,or to about 500 mA/cm², and these ranges may also apply to thoseembodiments referred to as providing “at least 100 mA/cm².”

The term “current efficiency” (I_(EFF)) may be described as the ratio ofthe total charge produced upon discharge of the system to the totalcharge passed upon charge. In some embodiments, the charge produced ondischarge or passed on charge can be measured using standardelectrochemical coulomb counting techniques well known to those ofordinary skill in the art. Without being bound by the limits of anytheory, the current efficiency may be a function of the state of chargeof the flow battery. In some non-limiting embodiments the currentefficiency can be evaluated over an SOC range of about 35% to about 60%.

The term “energy density” refers to the amount of energy that may bestored, per unit volume, in the active materials. Energy density, asused herein, refers to the theoretical energy density of energy storageand may be calculated by Equation 4:

Energy density=(26.8A-h/mol)×OCV×[e ⁻]  (4)

where OCV is the open circuit potential at 50% state of charge, asdefined above, (26.8 A-h/mol) is Faraday's constant, and [e⁻] is theconcentration of electrons stored in the active material at 99% state ofcharge. In the case that the active materials largely comprise an atomicor molecular species for both the positive and negative electrolyte,[e⁻] may be calculated as:

[e ⁻]=[active materials]×n/2  (5)

where [active materials] is the concentration (mol/L or M) of the activematerial in either the negative or positive electrolyte, whichever islower, and n is the number of electrons transferred per molecule ofactive material. The related term “charge density” refers to the totalamount of charge that each electrolyte may contain. For a givenelectrolyte:

Charge density=(26.8A-h/mol)×[active material]×n  (6)

where [active material] and n are as defined above.

The term “energy efficiency” may be described as the ratio of the totalenergy produced upon discharge of the system to the total energyconsumed upon charge. The energy efficiency (RT_(EFF)) may be computedby Equation 7:

RT _(EFF) =V _(EFF,RT) ×I _(EFF)  (7)

As used herein, the term “evolution current” describes the portion ofthe electrical current applied in an energized flow batteryconfiguration which is associated with the evolution (generation) of aparticular chemical species. In the current context, then, when asufficient overpotential vide infra) is applied in a flow battery suchthat either or both oxygen evolves at the positive electrode or hydrogenevolves at the negative electrode, that portion of the currentassociated with the evolution of oxygen or hydrogen is the oxygenevolution current or hydrogen evolution current, respectively.

In certain preferred embodiments, there is no current associated withhydrogen evolution, oxygen evolution, or both hydrogen and oxygenevolution. This may occur when the positive half-cell is operating at apotential less than the thermodynamic threshold potential or thethreshold overpotential of the positive electrode (i.e., no oxygenproduced; see explanation of terms below) or the negative half-cell cellis operating at a potential more positive than the thermodynamicthreshold potential or the threshold overpotential of the negativeelectrode (i.e., no hydrogen produced), or both. In separateembodiments, the batteries operates within 0.3 V, within 0.25 V, within0.2 V, within 0.15 V, or within 0.1 V of either the thermodynamicthreshold potential or the threshold overpotential of the respectivepositive or negative electrodes.

In embodiments wherein gas is evolved, the portion of current associatedwith gas evolution (either hydrogen or oxygen or both) is suitably lessthan about 20%, less than about 15%, less than about 10%, less thanabout 5%, less than about 2%, or less than about 1% of the total appliedcurrent. Lower gas evolution currents are considered particularlysuitable for battery (cell or cell stack) efficiencies.

The term “excluding” refers to the ability of a separator to not allowcertain ions or molecules to flow through the separator and typically ismeasured as a percent.

The term “mobile ion” is understood by those skilled in the art ofelectrochemistry and is meant to comprise the ion which is transferredbetween the negative and positive electrode during operation of theelectrochemical energy storage system. The term “mobile ion” may alsorefer to as an ion that carries at least at least 80% of the ioniccurrent during charger/discharge.

As used herein, the terms “negative electrode” and “positive electrode”are electrodes defined with respect to one another, such that thenegative electrode operates or is designed or intended to operate at apotential more negative than the positive electrode (and vice versa),independent of the actual potentials at which they operate, in bothcharging and discharging cycles. The negative electrode may or may notactually operate or be designed or intended to operate at a negativepotential relative to the reversible hydrogen electrode. The negativeelectrode is associated with the first aqueous electrolyte and thepositive electrode is associated with the second electrolyte, asdescribed herein.

The term “overpotential” is well understood by those skilled in the artof electrochemistry and is defined by the difference in voltage betweenan electrode during operation of an electrochemical cell and the normalhalf-cell potential of that electrode, as defined by the Nernstequation. Without being bound by theory, the term overpotential is meantto describe the energy, in excess of that required by thermodynamics, tocarry out a reaction at a given rate or current density. The term“overpotential” also describes a potential more positive than thethermodynamic onset voltage for oxygen evolution from water at thepositive electrode and more negative than the thermodynamic onsetvoltage for hydrogen evolution from water at the negative electrode.

Similarly, as used herein, the term “threshold overpotential” refers tothe overpotential at which either hydrogen or oxygen gas begins toevolve at the respective electrode. Note that an electrochemical systemcomprising “imperfect” (i.e., less than ideal catalytically) electrodescan be operated in three regions: (a) at a potential “below” thethermodynamic onset potential (i.e., more positive than thethermodynamic onset potential of the negative electrode and morenegative than the thermodynamic onset potential of the positiveelectrode; no gas evolving so no gas evolution current); (b) at apotential between the thermodynamic threshold potential and thresholdoverpotential (no gas evolving and still no evolution current); and (c)beyond the threshold overpotential (gas evolving and exhibiting a gasevolution current). Such threshold overpotentials can be identified bythose skilled in the art for a given system, for example, by measuringgas evolution as a function of applied half-cell potential (using e.g.,a mass spectrometer), in the presence or absence of an electroactivematerial. See also below.

The gas evolution threshold potentials are also affected by the natureof the electrolytes. Certain chemicals are known to inhibit theevolution of hydrogen and oxygen in electrolytic cells, either becauseof some activity in the bulk electrolyte or because of their ability tocoat or otherwise deactivate their respective electrodes; for example,macromolecules or oligomers or salts, such as chloride or phosphate, onPt surfaces. Accordingly, in certain embodiments, then, either the firstor second or both first and second electrolytes comprise at least onecompound increases the hydrogen or oxygen threshold overpotential of thesystem, respectively.

As used herein, the terms “regenerative fuel cell” or “reversible fuelcell” or “flow battery” or “flow energy device” connote the same orsimilar type of device, which utilizes the same battery configuration(including cell or cell stack) for both energy storage and energygeneration.

The term “reversible hydrogen electrode,” or RHE, is used in itsconventional meaning. That is, a reversible hydrogen electrode (RHE) isa reference electrode. The potential of the RHE, E(RHE) corresponds tothe potential for Equation 8:

2H ⁺+2e ⁻

H ₂  (8)

When the reaction of Equation 8 is carried out at equilibrium at a givenpH and 1 atm H₂. This potential can be reference to a normal hydrogenelectrode, E(NHE), by the following relation:

E(RHE)=E(NHE)−0.059×pH=0.0 V−0.059×pH  (9)

where E(NHE) is the potential for the normal hydrogen electrode (NHE=0.0V), defined as the potential for the reaction of Equation 8 at standardstate (1M H⁺, 1 atm H₂). Thus a potential of 0 V vs. RHE corresponds toa voltage of 0 V vs. NHE at pH 0 and −0.413 V vs. NHE at pH 7.

The term “selectivity” is well known to those of ordinary skill in theart of electrochemistry and refers to the ability of a membrane to allowa ratio of the movement of mobile ions to active materials through amembrane. For example, a membrane that allows a 50:1 ratio of mobileions to active materials to pass through would have a selectivity of 50.

The terms “separator” and “membrane” refer to an ionically conductive,electrically insulating material disposed between the positive andnegative electrode of an electrochemical cell.

The polymer electrolytes useful in the present disclosure may be anionor cation conducting electrolytes. Where described as an “ionomer,” theterm refers to a polymer comprising both electrically neutral and afraction of ionized repeating units, wherein the ionized units arependant and covalently bonded to the polymer backbone. The fraction ofionized units may range from about 1 mole percent to about 90 molepercent, but may be further categorized according to their ionized unitcontent. For example, in certain cases, the content of ionized units areless than about 15 mole percent; in other cases, the ionic content ishigher, typically at least about 80 mole percent. In still other cases,the ionic content is defined by an intermediate range, for example in arange of about 15 to about 80 mole percent. Ionized ionomer units maycomprise anionic functional groups comprising carboxylates, sulfonates,phosphonates, salts of a carboxy acid, sulfonic acid, phosphonic acid,and the like. These functional groups can be charge balanced by, mono-,di-, or higher-valent cations, such as alkali or alkaline earth metals.Ionomers may also include polymer compositions containing attached orembedded quaternary ammonium, sulfonium, phosphazenium, and guanidiniumresidues or salts. The polymers useful in the present disclosure maycomprise highly fluorinated or perfluorinated polymer backbones. Certainpolymer electrolytes useful in the present disclosure include copolymersof tetrafluoroethylene and one or more fluorinated, acid-functionalco-monomers, which are commercially available as NAFION™ perfluorinatedpolymer electrolytes from E. I. du Pont de Nemours and Company,Wilmington Del. Other useful perfluorinated electrolytes comprisecopolymers of tetrafluoroethylene (TFE) and FSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂.

The term “stack” or “cell stack” or “electrochemical cell stack” refersto a collection of individual electrochemical cells that are inelectrically connected. The cells may be electrically connected inseries or in parallel. The cells may or may not be fluidly connected.

The term “state of charge” (SOC) is well understood by those skilled inthe art of electrochemistry, energy storage, and batteries. The SOC isdetermined from the concentration ratio of reduced to oxidized speciesat an electrode (X_(red)/X_(ox)). For example, in the case of anindividual half-cell, when X_(red)═X_(ox) such that X_(red)/X_(ox)=1,the half-cell is at 50% SOC, and the half-cell potential equals thestandard Nernstian value, E°. When the concentration ratio at theelectrode surface corresponds to X_(red)/X_(ox)=0.25 orX_(red)/X_(ox)=0.75, the half-cell is at 25% and 75% SOC respectively.The SOC for a full cell depends on the SOCs of the individual half-cellsand in certain embodiments the SOC is the same for both positive andnegative electrodes. Measurement of the cell potential for a battery atOCV, and using Equations 2 and 3 the ratio of X_(red)/X_(ox) at eachelectrode can be determined, and therefore the SOC for the batterysystem.

The term “supporting electrolyte” is well-known in the arts ofelectrochemistry and energy storage, and is intended to refer to anyspecies which is redox inactive in the window of electric potential ofinterest and aids in supporting charge and ionic conductivity. In thepresent case, a supporting electrolyte does not substantially compromisethe solubility of the coordination compound or complex. Non-limitingexamples include salts comprising an alkali metal, ammonium ionincluding an ammonium ion partially or wholly substituted by alkyl oraryl groups, halide (e.g., Cl⁻, Br⁻, I⁻), chalcogenide, phosphate,hydrogen phosphate, phosphonate, nitrate, sulfate, nitrite, sulfite,perchlorate, tetrafluoroborate, hexafluorophosphate, or a mixturethereof, and others known in the art.

The term “voltage efficiency” may be described as the ratio of theobserved electrode potential, at a given current density, to thehalf-cell potential for that electrode (x 100%), wherein the half-cellpotential is calculated as described above. Voltage efficiencies can bedescribed for a battery charging step, a discharging step, or a “roundtrip voltage efficiency”. The round trip voltage efficiency (V_(EFF,RT))at a given current density can be calculated from the cell voltage atdischarge (V_(Discharge)) and the voltage at charge (V_(charge)) usingEquation 10:

V _(EFF,RT) =V _(Discharge) /V _(Charge)×100%  (10)

Exemplary Operating Characteristics

The present disclosure provides a variety of technical features of thedisclosed systems and methods. It should be understood that any one ofthese features may be combined with any one or more other features. Forexample, a user might operate a system featuring an electrolyte thatincludes an organic active material (e.g., a quinone), wherein thatelectrode has a pH of about 3. Such a system might also feature amembrane separator having a thickness of about 35 microns. It should befurther understood that the present disclosure is not limited to anyparticular combination or combinations of the following features.

The present invention also provides methods of operating the flowbatteries described herein, each method comprising charging said batteryby the input of electrical energy or discharging said battery by theremoval of electrical energy. Further embodiments provide applying apotential difference across the first and second electrode, with anassociated flow of electrons, so as to: (a) reduce the first redoxactive material while oxidizing the second redox active material; or (b)oxidize the first redox active material while reducing the second redoxactive material; or (c) both (a) and (b). Complementary methods providethose where each method comprises applying a potential difference acrossthe first and second electrode so as to: (a) oxidize the first redoxactive metal-ligand coordination compound; or (b) reduce the secondredox active metal-ligand coordination compound; or (c) both (a) and(b).

In traditional flow battery operation, mobile ions comprise proton,hydronium, or hydroxide. In various embodiments of the presentdisclosure, one may transportions other than proton, hydronium, orhydroxide (e.g., when these ions are present in comparatively lowconcentration, such as below 1 M). Separate embodiments of these methodsof operating a flow battery include those wherein the mobile ion doesnot consist essentially of protons, hydronium, or hydroxide. In theseembodiments, less than 50% of the mobile ions comprise protons,hydronium, or hydroxide. In other embodiments, less than about 40%, lessthan about 30%, less than about 20%, less than about 10%, less thanabout 5%, or less than about 2% of the mobile ions comprise protons,hydronium, or hydroxide. Exemplary mobile ions in these embodimentsinclude alkali metal or alkaline earth metal cations (especially Li⁺,Na⁺, K⁺, Mg²⁺, Ca²⁺ or Sr²⁺).

In some embodiments of the present disclosure, it is advantageous tooperate between pH 1 and 13 (e.g., to enable active material solubilityand/or low system cost). In this case one or both electrolytes ischaracterized as having a pH of between about 1 and about 13, or betweenabout 2 and about 12, or between about 4 and about 10, or even betweenabout 6 and about 8. In other embodiments, at least one of theelectrolytes has a pH in a range of from about 9 to about 13, from about8 to about 12, from about 10 to about 12, or from about 10.5 to about11.5. For the most part, the compounds described herein comprisingcatecholate or pyrogallate are stable and operable at pH's within eachof the ranges described herein. In some embodiments, the pH of theelectrolyte may be maintained by a buffer. Typical buffers include saltsof phosphate, borate, carbonate, silicate, trisaminomethane (Tris),4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),piperazine-N,N′-bis(ethanesulfonic acid) (PIPES), and combinationsthereof. A user may add an acid (e.g., HCl, HNO₃, H₂SO₄ and the like), abase (NaOH, KOH, and the like), or both to adjust the pH of a givenelectrolyte as desired.

In some embodiments, the pH of the first and second electrolytes areequal or substantially similar; in other embodiments, the pH of the twoelectrolytes differ by a value in the range of about 0.1 to about 2 pHunits, about 1 to about 10 pH units, about 5 to about 12 pH units, about1 to about 5 pH units, about 0.1 to about 1.5 pH units, about 0.1 toabout 1 pH units, or about 0.1 to about 0.5 pH units. In this context,the term “substantially similar,” without further qualification, isintended to connote that the difference in pH between the twoelectrolytes is about 1 pH unit or less. Additional optional embodimentsprovide that the pH difference is about 0.4 or less, about 0.3 or less,about 0.2 or less, or about 0.1 pH units or less.

The disclosed systems and methods may also comprise active materials andmembrane ionomers which are charged. The term “charge” in refers to the“net charge” or total charge associated with an active material orionomer moiety. The charged species may be anionic or cationic. Incertain desired embodiments of the present disclosure it is advantageousfor the active materials and membrane ionomers to comprise charges ofthe same sign (e.g. to prevent transfer of the active material acrossthe membrane).

In specific embodiments, both the first and second ionically chargedredox active materials and their respective oxidized or reduced formsare negatively charged, and the ion selective membrane having astationary phase that also has a net negative charge, so as to beselectively permeable to cations to the substantial exclusion of thenegatively charged redox active materials. The first and second redoxactive materials and their respective oxidized or reduced forms mayindependently exhibit charges in a range of −2 to −5. The term“substantial exclusion” refers to the ability of the membrane to limitthe molar flux of ions passing through the membrane attributable to thefirst or second ionically charged redox active material to less thanabout 3% of the total ion flux during the operation of the flow battery.In related independent embodiments, the flux of ions attributable to thefirst or second ionically charged redox active material is about 5% orless, about 2% or less, about 1% or less, about 0.5% or less, about 0.2%or less, about 0.1% of less, about 0.01% or less, or about 0.001% orless of the total ion flux during the operation of the flow battery.

In other embodiments, both the first and second ionically charged redoxactive materials and their respective oxidized or reduced forms arepositively charged, the ion selective membrane having a stationary phasethat also has a net positive charge, so as to be selectively permeableto anions to the substantial exclusion of the positively charged redoxactive materials. The first and second redox active materials and theirrespective oxidized or reduced forms may independently exhibit chargesin a range of +2 to +5 over the respective potential ranges. The term“substantial exclusion” is as described above.

The ability to measure the molar flux of the charged redox activematerial through the membrane during the operation of the flow batterymay be conveniently measured for those systems in which each electrolytecomprises a redox active material based on a different metal such asprovided in some embodiments described here (e.g., iron in the positiveelectrolyte and titanium in the negative electrolyte). This may be doneby (a) operating such a cell at a fixed temperature (typically ambientroom, but also super-ambient, temperatures) for a prescribed period oftime (depending on the rate of flux, for example, 1 hour), (b) measuringand quantifying the amount of metal which has passed through themembrane from the source to second electrolyte (using, for example,atomic absorption spectroscopy, inductively coupled plasma, ionchromatography, or other suitable method), and (c) comparing that amountof metal ions with the amount of mobile ion which has passed through themembrane, corresponding to the total electrons which have passed overthat period of time. By measuring the flux as a function of time andtemperature, and membrane thicknesses, it is also possible to calculatethe thermodynamic parameters associated with this particular system, andpredict longevity of the system.

Systems and methods according to the present disclosure also featureactive materials comprising metal-ligand coordination compounds.Metal-ligand coordination compounds may be present at, e.g., aconcentration of at least about 0.25 M, at least about 0.35 M, at leastabout 0.5 M, at least about 0.75 M, at least about 1 M, at least about1.25 M, at least about 1.5 M, at least about 2 M, for example as high as3 M, 4 M, or 5M.

The metal ligand coordination compound may be further characterized withrespect to the nature of the oxidizable or reducible species. Forexample, in some cases, the redox potential of the metal ligandcoordination compound may be defined by transitions entirely within themetal center—i.e., the redox potential is defined by the accessibilityof and energies associated with transitions between various valencestates within the metal. In other cases, the oxidation/reduction may belocalized within the ligand system. In still other cases, theoxidation/reduction may be distributed throughout the entire redoxactive complex, such that both the metal and the ligand system sharingin the distribution of charge.

In particular embodiments of the present disclosure, the metal ligandcoordination compound may comprise ligands which are mono-, bi-, tri-,or multidentate. Monodentate ligands bind to metals through one atom,whereas bi-, tri-, or multidentate ligands bind to metals through 2, 3,or more atoms, respectively. Examples of monodentate ligands includehalogens (F⁻, Cl⁻, Br⁻, I⁻), cyanide (CN⁻), carbonyl or carbon monoxide(CO), nitride (N³⁻), oxo (O²⁻), hydroxo (OH⁻), sulfide (S²⁻), pyridine,pyrazine, and the like. Other types of ligand bonding moieties includeamino groups (NR₃), amido groups (NR₂), imido groups (NR), alkoxy groups(R—CO⁻), siloxy (R—SiO⁻), thiolate (R—S⁻), and the like, which maycomprise mono-, bi-, tri-, or multidentate ligands. Examples ofbidentate ligands include catechol, bipyridine, bipyrazine,ethylenediamine, diols (including ethylene glycol), and the like.Examples of tridentate ligands include terpyridine, diethylenetriamine,triazacyclononane, trisaminomethane, and the like.

The disclosed systems and methods may feature electrochemical cellseparators and/or membranes that have certain characteristics. In thisdisclosure, the terms membrane and separator are used interchangeably.The membranes of the present disclosure may, in some embodiments,feature a membrane separator having a thickness of about 500 microns orless, about 300 microns or less, about 250 microns or less, about 200microns or less, about 100 microns or less, about 75 microns or less,about 50 microns or less, about 30 microns or less, about 25 microns orless, about 20 microns or less, about 15 microns or less, or about 10microns or less, for example to about 5 microns, and where the phrase“100 microns or less” is used, separate embodiments include those usingthese ranges.

Separators are generally categorized as either solid or porous. Solidmembranes typically comprise an ion-exchange membrane, wherein anionomer facilitates mobile ion transport through the body of thepolymer. The facility with which ions conduct through the membrane canbe characterized by a resistance, typically an area resistance in unitsof ohm-cm². The area resistance is a function of inherent membraneconductivity and the membrane thickness. Thin membranes are desirable toreduce inefficiencies incurred by ion conduction and therefore can serveto increase voltage efficiency of the energy storage device. Activematerial crossover rates are also a function of membrane thickness, andtypically decrease with increasing membrane thickness. Crossoverrepresents a current efficiency loss that must be balanced with thevoltage efficiency gains by utilizing a thin membrane.

Porous membranes are non-conductive membranes that allow charge transferbetween two electrodes via open channels filled with conductiveelectrolyte. Porous membranes are permeable to liquid or gaseouschemicals. This permeability increases the probability of chemicalspassing through porous membrane from one electrode to another causingcross-contamination and/or reduction in cell energy efficiency. Thedegree of this cross-contamination depends on, among other features, thesize (the effective diameter and channel length), and character(hydrophobicity/hydrophilicity) of the pores, the nature of theelectrolyte, and the degree of wetting between the pores and theelectrolyte. Certain embodiments also provide that the first or thesecond or both the first and second metal ligand coordination compoundsare characterized as having a hydrodynamic diameter and separator ischaracterized as having a mean pore size, wherein the hydrodynamicdiameter of the coordination compound is larger than the mean pore sizeof the separator.

Such ion-exchange separators may also comprise membranes, which aresometimes referred to as polymer electrolyte membranes (PEMs) or ionconductive membranes (ICMs). The membranes according to the presentdisclosure may comprise any suitable polymer, typically an ion exchangeresin, for example comprising a polymeric anion or cation exchangemembrane, or combination thereof. The mobile phase of such a membranemay comprise, and/or is responsible for the primary or preferentialtransport (during operation of the battery) of at least one mono-, di-,tri-, or higher valent cation and/or mono-, di-, tri-, or higher valentanion, other than protons or hydroxide ions.

Additionally, substantially non-fluorinated membranes that are modifiedwith sulfonic acid groups (or cation exchanged sulfonate groups) mayalso be used. Such membranes include those with substantially aromaticbackbones, e.g., poly-styrene, polyphenylene, bi-phenyl sulfone (BPSH),or thermoplastics such as polyetherketones or polyethersulfones.Examples of ion-exchange membranes comprise Nafion.

Battery-separator style porous membranes, may also be used. Because theycontain no inherent ionic conduction capability, such membranes aretypically impregnated with additives in order to function. Thesemembranes are typically comprised of a mixture of a polymer, andinorganic filler, and open porosity. Suitable polymers include thosechemically compatible with the electrolytes of the presently describedsystems, including high density polyethylene, polypropylene,polyvinylidene difluoride (PVDF), or polytetrafluoroethylene (PTFE).Suitable inorganic fillers include silicon carbide matrix material,titanium dioxide, silicon dioxide, zinc phosphide, and ceria and thestructures may be supported internally with a substantiallynon-ionomeric structure, including mesh structures such as are known forthis purpose in the art.

The open circuit potential (OCV) of an electrochemical cell is arelevant operating characteristic of electrochemical energy storagesystems. In certain embodiments, the OCV may be comparatively large(e.g. at least 1 V, and upwards of 2 V, 3 V, or 4 V). Such comparativelylarge open circuit potentials are known to enable high cell voltageefficiencies, high AC-AC conversion efficiencies, high energy storagedensities, and low system costs. Traditional flow batteries with aqueouselectrolytes and soluble active materials may operate with an OCV lessthan about 1.2 V. An electrochemical cell according to the presentdisclosure is suitably characterized by an open circuit potential of atleast about 1.4 V.

The present disclosure presents exemplary cyclic voltammetry data forseveral metal ligand coordination compound couples under a range ofconditions (see Tables 2 and 3, and Example 7, vide infra). Inconsidering these (or other) sets of half-cell couples, certainembodiments provide that the cells comprise those pairs of metal ligandcoordination compounds whose couples provide large open circuitpotential, while capable of operating at potentials that are within thepotentials associated with the generation of hydrogen and oxygen derivedfrom the electrolysis of water (i.e., so as to operate at potentialswhere the generation of a hydrogen or oxygen evolution current isminimized or avoided). In certain embodiments, these half-cell couplesare chosen to provide large open circuit voltages while operating at orbelow a half-cell potential of 0 V at the negative electrode and at orabove a half-cell potential of 1.23 V at the positive electrode, wherethe half-cell potentials are with respect to a reversible hydrogenelectrode. Through judicious choice of electrode materials which exhibitpoor catalytic activity, e.g., an allotrope of carbon or a metal oxide,it is possible to provide systems having large overpotentials, so as todrive the OCV to values higher than the thermodynamic limit of 1.23 Vwithout hydrogen or oxygen evolution. For example, experiments show (andas reflected in Table 3, vide infra) the Ti^(4+/3+) (cat)₃ ^(2−/3−) andAl(cit)₂(cat)^(2−/3−) pair of couples can exhibit an OCV of 1.70 V usingcarbon electrodes.

In some embodiments, the open circuit voltage (OCV) of the flow batteryis at least about 1.2 volts, at least about 1.3 V, at least about 1.4 V,at least about 1.5 V, at least about 1.6 V, at least about 1.7 V, atleast about 1.8 V, at least about 1.9 V, or at least about 2 V, forexample to about 3 V or 4V. As described above, higher open circuitvoltages are associated with higher power densities.

Systems and methods according to the present disclosure may exhibit aparticular current density at a given round trip voltage efficiency.Methods for determining current density at a given round trip voltageefficiency are known to those skilled in the art of electrochemistry andelectrochemical energy storage.

To serve as a metric for electrochemical cell performance, a specifiedcurrent density must be linked to a measured voltage efficiency. Highercurrent densities for a given round trip voltage efficiency enable lowercost electrochemical cells and cell stacks. In certain embodiments, itis desired to operate a flow battery with a current density of at leastabout 50 mA/cm² at V_(EFF,RT) of at least about 50%. In otherembodiments, the current density will be at least about 50 mA/cm² atV_(EFF,RT) of at least about 60%, at least about 75%, at least about85%, or at least about 90%. In other embodiments, the current densitywill be at least 100 mA/cm² at V_(EFF,RT) of at least about 50%, atleast about 60%, at least about 75%, at least about 85%, at least about90% and the like. In other embodiments, the current density will be atleast 200 mA/cm² at V_(EFF,RT) of at least about 50%, at least about60%, at least about 75%, at least about 85%, at least about 90%, andabove.

Electrolytes that include an organic active material, either in theabsence or presence of metal coordination, are considered suitable forone or both half-cells of the disclosed systems and methods. Suitableorganic active materials include carbon, aromatic hydrocarbons,including quinones, hydroquinones, viologens, pyridinium, pyridine,acridinium, catechol, other polycyclic aromatic hydrocarbons, and thelike. Suitable organic active materials may also include sulfur,including thiol, sulfide, and disulfide moieties. Suitable organicactive materials may be soluble in water in concentrations at leastabout 0.1 M, at least about 0.5 M, at least about 1 M, at least about1.5 M, at least about 2 M, and above, for example, to about 2M, about 3M, about 4 M, or about 5 M.

The disclosed systems and methods may also be characterized in terms oftheir half-cell potentials. Both the negative and positive electrode mayexhibit a half-cell potential. An electrochemical cell according to thepresent disclosure may, in some embodiments, have a half-cell potentialfor the negative electrode less than about 0.5 V vs. RHE, less thanabout 0.2 V vs. RHE, less than about 0.1 V vs. RHE, less than about 0.0V vs. RHE, less than about −0.1 V vs. RHE, less than about −0.2 V vs.RHE, less than about −0.3 V vs. RHE, less than about −0.5 V vs. RHE, forexample, to about −2 V vs. RHE. An electrochemical cell according to thepresent disclosure may, in some embodiments, have a half-cell potentialfor the positive electrode at least about 0.5 V vs. RHE, at least about0.7 V vs. RHE, at least about 0.85 V vs. RHE, at least about 1.0 V vs.RHE, at least about 1.1 V vs. RHE, at least about 1.2 V vs. RHE, atleast about 1.3 V vs. RHE, at least about 1.4 V vs. RHE and the like,for example, to about 2 V vs. RHE.

The disclosed systems and methods may also be characterized in terms oftheir energy density, as defined above. Flow batteries of the presentdisclosure may operate with an energy density of about 5 Wh/L, betweenabout 5 Wh/L and about 15 Wh/L, between about 10 Wh/L and about 20 Wh/L,between about 20 Wh/L and about 30 Wh/L, between about 30 and about 40Wh/L, between about 25 Wh/L and about 45 Wh/L, and above 45 Wh/L, forexample to about 50 Wh/L, to about 60 Wh/L, or to about 70 Wh/L.

Among the many embodiments considered within the scope of the presentinvention are these:

Embodiment 1. A flow battery comprising:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte; and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte; wherein:

(a) each of the first and second redox active materials comprises ametal-ligand coordination compound that independently exhibitssubstantially reversible electrochemical kinetics; or

(b) the first, second, or both redox active materials comprise a metalligand coordination compound in concentrations of about 0.75 M orhigher; or

(c) the first, second, or both first and second redox active materialseach comprise a metal ligand coordination compounds and said flowbattery is capable of operating with a current density of about 100mA/cm² or higher and a round trip voltage efficiency of about 70% orhigher; or

(d) the first, second, or both redox active materials comprise a metalligand coordination compound and the separator has a thickness of lessthan 100 μm; or

(e) the first, second, or both redox active materials comprise a metalligand coordination compound and wherein the energy density of theelectrolytes is about 30 Wh/L or higher; or

(f) the flow battery comprises any combination of (a) through (e).

Embodiment 2. A flow battery comprising:

a first aqueous electrolyte comprising a first redox active material;

a second aqueous electrolyte comprising a second redox active material;

a first electrode in contact with said first aqueous electrolyte;

a second electrode in contact with said second aqueous electrolyte and

a separator disposed between said first aqueous electrolyte and saidsecond aqueous electrolyte;

wherein the first or second redox active material, or both the first andsecond redox active materials comprise a metal ligand coordinationcompound having a formula comprising M(L1)_(x)(L2)_(y)(L3)_(z) ^(m),where M is independently a non-zero valent metal or metalloid of Groups2-16, including lanthanides and actinides,

wherein x, y, and z are independently 0, 1, 2, or 3, and 1≦x+y+z≦3;

m is independently −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, or 5; and

L1, L2, and L3 are each independently ascorbate, citrate, a glycolate,gluconate, glycinate, α-hydroxyalkanoate, β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, a polyol, sarcosinate,salicylate, lactate, or a compound having structure according to FormulaI, or an oxidized or reduced form thereof:

wherein

Ar is a 5-20 membered aromatic moiety, optionally comprising one of moreO, N, or S heteroatoms;

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof, X₁ andX₂ being positioned ortho to one another;

R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆ alkyl, C₁₋₆alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, a boric acid ora salt thereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano,halo, hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol;

R′ is independently H or C₁₋₃ alkyl; and

n is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

Embodiment 3. The flow battery of Embodiment 2, wherein the first andsecond redox active materials comprise a metal ligand coordinationcompound.

Embodiment 4. The flow battery of Embodiment 2 or 3, wherein (a) x=3,y=z=0; (b) x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2,y=z=0; or (f) x=1, y=z=0.

Embodiment 5. The flow battery of any one of Embodiments 1 to 4, whereinthe first aqueous electrolyte comprises a first metal-ligandcoordination compound and the second aqueous electrolyte comprises asecond metal-ligand coordination compound, wherein the first and secondmetal-ligand coordination compounds are different.

Embodiment 6. The flow battery of any one of Embodiments 1 to 5, whereinthe first, the second, or both the first and second metal-ligandcoordination compound comprises at least one ligand having a structureaccording to Formula I.

Embodiment 7. The flow battery of any one of Embodiments 1 to 6, whereinthe first, the second, or both of the redox-active metal ligandcoordination compounds comprises at least one ligand having a structureaccording to Formula IA, IB, or IC:

wherein

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof;

R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆ alkyl, aboric acid or a salt thereof, carboxy acid or a salt thereof, C₂₋₆carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol;

R′ is independently H or C₁₋₃ alkyl; and

n is 0-4.

Embodiment 8. The flow battery of Embodiment 7, wherein

X₁ and X₂ are both OH or an anion thereof;

R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid or a saltthereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano, halo,hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol; and

n is 1.

Embodiment 9. The flow battery of any one of Embodiments 1 to 8, wherethe first metal-ligand coordination compound comprises at least oneascorbate, a catecholate, citrate, a glycolate or polyol, gluconate,glycinate, α-hydroxyalkanoate, β-hydroxyalkanoate, γ-hydroxyalkanoate,malate, maleate, phthalate, sarcosinate, salicylate, or lactate ligand.

Embodiment 10. The flow battery of any one of Embodiments 1 to 9, wherethe second metal-ligand coordination compound comprises at least oneascorbate, a catecholate, citrate, a glycolate or polyol, gluconate,glycinate, α-hydroxyalkanoate, β-hydroxyalkanoate, γ-hydroxyalkanoate,malate, maleate, phthalate, sarcosinate, salicylate, or lactate ligand.

Embodiment 11. The flow battery of any one of Embodiments 1 to 10, wherethe first metal-ligand coordination compound comprises at least oneligand of Formula I, IA, IB, or IC.

Embodiment 12. The flow battery of any one of Embodiments 1 to 11, wherethe second metal-ligand coordination compound comprises at least oneligand of Formula I, IA, IB, or IC.

Embodiment 13. The flow battery of any one of Embodiments 1 to 12,wherein either the first or the second or both the first and secondmetal-ligand coordination compound comprises Al, Ca, Ce, Co, Cr, Fe, Mg,Mn, Mo, Si, Sn, Ti, W, Zn, or Zr.

Embodiment 14. The flow battery of any one of Embodiments 1 to 13,wherein the first metal-ligand coordination compound comprises Al³⁺,Ca²⁺, Ce⁴⁺, Co³⁺, Cr³⁺, Fe³⁺, Mg²⁺, Mn³⁺, Mo⁶⁺, Si⁴⁺, Sn⁴⁺, Ti⁴⁺, W⁶⁺,Zn²⁺, or Zr⁴⁺.

Embodiment 15. The flow battery of any one of Embodiments 1 to 14,wherein the second metal-ligand coordination compound comprises Al³⁺,Ca²⁺, Ce⁴⁺, Co³⁺, Cr³⁺, Fe³⁺, Mg²⁺, Mn³⁺, Mo⁶⁺, Si⁴⁺, Sn⁴⁺, Ti⁴⁺, W⁶⁺,Zn²⁺, or Zr⁴⁺.

Embodiment 16. The flow battery of any one of Embodiments 1 to 15,wherein the first metal-ligand coordination compound comprises Cr, Ti,or Fe.

Embodiment 17. The flow battery of any one of Embodiments 1 to 16,wherein the second metal-ligand coordination compound comprises Cr, Ti,or Fe.

Embodiment 18. The flow battery of any one of Embodiments 1 to 17,wherein the second metal-ligand coordination compound comprises an ironhexacyanide compound.

Embodiment 19. The flow battery of any one of Embodiments 1 to 18,wherein either the first or the second or both the first and secondmetal-ligand coordination compounds is characterized as having ahydrodynamic diameter and the separator is characterized as having amean pore size, wherein the hydrodynamic diameter of the coordinationcompound is larger than the mean pore size of the separator.

Embodiment 20. The flow battery of any one of Embodiments 1 to 19,wherein either or both of the first or the second metal-ligandcoordination compound are present in the first or second electrolyte,respectively, at a concentration of at least 0.75 M.

Embodiment 21. The flow battery of any one of Embodiments 2 to 20,wherein the first and second metal-ligand coordination compounds eachexhibits substantially reversible electrochemical kinetics.

Embodiment 22. The flow battery of any one of Embodiments 2 to 21,wherein the cell exhibits a round trip voltage efficiency of at least70%, when measured at 200 mA/cm².

Embodiment 23. The flow battery of any one of Embodiments 2 to 22,wherein the flow battery is capable of operating, or is operating, witha current density of about 100 mA/cm² or higher and a round trip voltageefficiency of about 70% or higher.

Embodiment 24. The flow battery of any one of Embodiments 2 to 23,wherein the separator has a thickness of less than less than 100 μm.

Embodiment 25. The flow battery of any one of Embodiments 2 to 24,wherein the energy density of the electrolytes is about 30 Wh/L orhigher.

Embodiment 26. The flow battery of any one of Embodiments 2 to 25,wherein the cell retains at least 70% round trip voltage efficiency whensubjected to 10 charge/discharge cycles.

Embodiment 27. The flow battery of Embodiment 1,

wherein the first, second, or first and second redox active materialcomprises a metal ligand coordination complex having a formulacomprising M(L1)_(x)(L2)_(y)(L3)_(z) ^(m), where x, y, and z areindependently 0, 1, 2, or 3 and 1≦x+y+z≦3; and where

M is Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, S, Sn, Ti, W, Zn, or Zr;

L1, L2, and L3 are each independently ascorbate, a catecholate, citrate,a glycolate or polyol, gluconate, glycinate, α-hydroxyalkanoate,β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, phthalate,sarcosinate, salicylate, lactate or a compound having structureaccording to Formula I, or an oxidized or reduced form thereof:

wherein

Ar is a 5-20 membered aromatic moiety, optionally comprising one of moreO, N, or S heteroatoms;

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof, X₁ andX₂ being positioned ortho to one another;

R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆ alkyl, C₁₋₆alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, a boric acid ora salt thereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano,halo, hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol;

R′ is independently H or C₁₋₃ alkyl;

and m is +1, 0, −1, −2, −3, −4, or −5.

Embodiment 28. The flow battery of Embodiment 27, where (a) x=3, y=z=0;(b) x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0;or (f) x=1, y=z=0.

Embodiment 29. The flow battery of Embodiment 1, wherein either thefirst or the second or both the first and second metal-ligandcoordination compound comprises at least one ligand having a structureaccording to Formula I.

Embodiment 30. The flow battery of Embodiment 28, wherein either or bothof the redox-active metal ligand coordination compounds comprises atleast one ligand having a structure according to Formula IA, IB, or IC:

wherein

X₁ and X₂ are independently —OH, —NHR′, —SH, or an anion thereof;

R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆ alkyl, aboric acid or a salt thereof, a carboxy acid or a salt thereof, C₂₋₆carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, a sulfonic acid ora salt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol;

R′ is independently H or C₁₋₃ alkyl; and

n is 0-4.

Embodiment 31. The flow battery of Embodiment 30, wherein

X₁ and X₂ are both OH or an anion thereof;

R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid or a saltthereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano, halo,hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol; and

n is 1.

Embodiment 32. The flow battery of any one of Embodiments 1 to 31, wherethe second metal-ligand coordination compound comprises at least onecatechol or pyrogallol ligand.

Embodiment 33. The flow battery of any one of Embodiments 1 to 32,wherein the second metal-ligand coordination compound is an ironhexacyanide compound.

Embodiment 34. The flow battery of any one of Embodiments 1 to 33,wherein either the first or the second or both the first and secondmetal-ligand coordination compounds is characterized as having ahydrodynamic diameter and the separator is characterized as having amean pore size, wherein the hydrodynamic diameter of the coordinationcompound is larger than the mean pore size of the separator.

Embodiment 35. A system comprising a flow battery of any one ofEmbodiments 1 to 34, and further comprising:

(a) a first chamber containing the first aqueous electrolyte and asecond chamber containing the second aqueous electrolyte;

(b) at least one electrolyte circulation loop in fluidic communicationeach electrolyte chamber, said at least one electrolyte circulation loopcomprising storage tanks and piping for containing and transporting theelectrolytes;

(c) control hardware and software; and

(d) an optional power conditioning unit.

Embodiment 36. The system of Embodiment 35, the system connected to anelectrical grid configured to provide renewables integration, peak loadshifting, grid firming, baseload power generation/consumption, energyarbitrage, transmission and distribution asset deferral, weak gridsupport, frequency regulation, or a combination thereof.

Embodiment 37. The system of Embodiment 35 or 36, the system configuredto provide stable power for remote camps, forward operating bases,off-grid telecommunications, or remote sensors.

Embodiment 38. A method of operating a flow battery of any one ofEmbodiments 1 to 34, said method comprising charging said battery by theinput of electrical energy or discharging said battery by the removal ofelectrical energy.

Embodiment 39. A method of charging a flow battery or system of any oneof Embodiments 1 to 37, with an associated flow of electrons, saidmethod comprising applying a potential difference across the first andsecond electrode, so as to:

(a) reduce the first redox active metal-ligand coordination compound; or

(b) oxidize the second redox active metal-ligand coordination compound;or

(c) both (a) and (b).

Embodiment 40. A method of discharging the flow battery or system of anyone of Embodiments 1 to 37, with an associated flow of electrons, saidmethod comprising applying a potential difference across the first andsecond electrode so as to:

(a) oxidize the first redox active metal-ligand coordination compound;or

(b) reduce the second redox active metal-ligand coordination compound;or

(c) both (a) and (b).

Example 1 General

The following Examples are provided to illustrate some of the conceptsdescribed within this disclosure. While each Example is considered toprovide specific individual embodiments of composition, methods ofpreparation and use, none of the Examples should be considered to limitthe more general embodiments described herein.

Example 1.1 Materials

Sodium hexacyanoferrate(II) decahydrate 99%, Na₄Fe(CN)₆.10H₂O; potassiumhexacyanoferrate(II) trihydrate 98+%, K₄Fe(CN)₆.3H₂O; potassiumhexacyanoferrate(III) ACS 99.0% min; K₃Fe(CN)₆; ethylene glycol,propylene glycol, glycerol, lactic acid (80-85 aqueous solution);glycine, glycolic acid (67% aqueous solution); maleic acid; malic acid;phthalic acid; salicylic acid; gluconic acid; citric acid; sarcosine;iron (III) sulfate; iron (III) chloride; titanium oxysulfate; manganese(II) sulfate; and chromium (III) sulfate were purchased from Alfa Aesar(Ward Hill, Mass.) as ACS grade or better unless specified above andwere used without additional purification. Ammoniumbislactatobishydroxytitanium (IV) was purchased from Sigma Aldrich (St.Louis, Mo.) as a 50% aq. solution and was used without furtherpurification. Potassium hexacyanochromate(III), K₃[Cr(CN)₆] andpotassium hexacyanomanganate(III), K₃-[Mn(CN)₆] were purchased fromSigma-Aldrich (St. Louis, Mo.) and used without additional purification.

Complexes could be synthesized by several methods. Homoleptictris-ligated complexes were most easily synthesized by stirring a 3:1aqueous mixture of ligand and metal salt while slowly adding an alkalimetal hydroxide solution until the pH was between 8 and 13, the typicalwindow of stability for the complexes of interest. Certain mixed ligandspecies, for example Ti(lactate)₂(salicylate), could also be synthesizedby this method.

Mono and bis α-hydroxy acid complexes of iron and titanium weresynthesized by the portion-wise addition of 2 equivalents of sodiumbicarbonate to stirred solutions of the metal sulfates (2-3 M) and theappropriate proportion of the appropriate ligand. For example, 6 mmol ofTiOSO₄ and 6 mmol of glycolic acid were stirred, and 12 mmol of NaHCO₃was added slowly, allowing gas evolution to subside between additions.The pH of the resulting solutions was about 3.5 for the solutions of ML₁and about 2 for the solutions of ML₂. The solubility of these complexesrelative to aquated metals is evidenced by the stability with respect toprecipitation of metal oxides of TiL₁ and TiL₂ solutions at such highpHs. In a control experiment where no ligand was added, wholesale andirreversible precipitation of TiO₂ was observed when more than 1equivalent of NaHCO₃ was added, corresponding to a pH of about 1.

Complexes with additional ligands could be synthesized by adding anappropriate amount of ML₁ or ML₂ solution synthesized as described inthe previous paragraph to a solution of the desired additional ligandmixed with a suitable base, such as potassium carbonate or potassiumhydroxide. Mixed ligand analogs of the Mn, Cr, Ti, and Fe compounds maybe prepared by similar reaction schemes.

Titanium bis-lactate L′ complexes could also be synthesized using(NH₄)₂Ti(lactate)₂(OH)₂ (available from Sigma Aldrich as a 50% solution)as a synthon. In this case, L′ (e.g., salicylic acid) was added, andafter about an hour of stirring, an aqueous solution of 2 eq. alkalimetal hydroxide was added to deprotonate ammonium, drive off ammoniaover the course of about 24 hours of stirring uncapped in a fume hood,and provide the desired metal complex as a sodium/potassium salt, e.g.,NaKTi(lactate)₂(salicylate).

Disodium titanium(IV) triscatecholate, Na₂Ti(catecholate)₃ wassynthesized by a modification of a procedure described by Davies, seeDavies, J. A.; Dutramez, S. J. Am. Ceram. Soc. 1990, 73. 2570-2572, fromtitanium(IV) oxysulfate and pyrocatechol. Sodium hydroxide was used inplace of ammonium hydroxide to obtain the sodium salt. Sodium potassiumtitanium(IV) trispyrogallate, NaKTi(pyrogallate)₃ was made analogously,first as the ammonium salt, (NH₄)Ti(pyrogallate)₃, and subsequentlyconverted to the sodium potassium salt by heating in a mixture ofaqueous sodium hydroxide and aqueous potassium hydroxide.

The mixed ligand titanium complexes sodium potassium titanium(IV)biscatecholate monopyrogallate, sodium potassium titanium(IV)biscatecholate-monolactate, sodium potassium titanium (IV)biscatecholate monogluconate, sodium potassium titanium(IV)biscatecholate monoascorbate, and sodium potassium titanium(IV) biscatecholate monocitrate were made from a titanium catecholate dimer,Na₂K₂-[TiO(catecholate)]₂. For the synthesis of the tetrapotassium saltsee Borgias, B. A.; Cooper, S. R.; Koh, Y. B.; Raymond, K. N. Inorg.Chem. 1984, 23, 1009-1016. A one-to-one mixture of titanium dimer withthe desired chelate (pyrogallol, lactic acid, gluconic acid, ascorbicacid, or citric acid) gave the mixed ligand species. Sodium potassiumtitanium(IV) monocatecholate monopyrogallate monolactate was made in asimilar fashion by addition of both pyrogallol and lactic acid to thecatecholate containing dimer. Mixed ligand analogs of the Al, Cr, Fe,and Mn compounds may be prepared by similar reaction schemes. Thestructures of several of the titanium compounds were confirmed by massspectroscopy (see Table 1). Mixed ligand analogs of the Al, Cr, Fe, andMn compounds may be prepared by similar reaction schemes.

TABLE 1 Mass spectroscopy data for selected compound* Mass (m/z)Calc'd/Obs'd Ti(catecholate)₃ ²⁻ 186.0080/186.0 Ti(pyrogallate)₃ ²⁻210.0038/210.0 Ti(catecholate)₂(pyrogallate)²⁻ 194.0055/194.0Ti(catecholate)₂(ascorbate)²⁻ 219.0057/219.0Ti(catecholate)₂(gluconate)²⁻ 229.0188/229.0 Ti(catecholate)₂(lactate)²⁻176.0055/176.0 *Mass spectrometry data were obtained on an Agilent 6150Bsingle quadrupole LC/MS in the negative ion mode with electrosprayionization (ESI). Aqueous solution samples of the metal ligand complexwere diluted in methanol and introduced to the mass spectrometer ionizerby direct injection using a syringe pump. The reported m/z peaks in eachcase are for the dianions, z = −2.

Sodium potassium iron(III) triscatecholate,Na_(1.5)K_(1.5)Fe(catecholate)₃ was prepared according to the procedureoutline by Raymond et. al., see Raymond, K. N.; Isied, S. S., Brown, L.D.; Fronczek, F. R.; Nibert, J. H. J. Am. Chem. Soc. 1976, 98,1767-1774. The only modification was the use of a mixture of sodiumhydroxide and potassium hydroxide as the excess base in place ofpotassium hydroxide.

Sodium titanium(IV) triscitrate, Na₄Ti(citrate)₃, was synthesized byanalogy to the method used for sodium titanium(IV) tricatecholatedescribed above except using citric acid in place of catechol. Thesestarting materials were obtained from Alfa Aesar (Ward Hill, Mass.),were of reagent grade or better, and were used as received.

Sodium aluminum(III) biscitrate monocatecholate,Al(citrate)₂(catecholate), was synthesized in analogy to the method usedfor sodium titanium(IV) tricatecholate described above except using twoequivalents of citric acid and one equivalent of catechol to a solutionof aluminum(III) sulfate. These starting materials were obtained fromAlfa Aesar (Ward Hill, Mass.), were of reagent grade or better, and wereused as received.

Example 1.2 Cyclic Voltammetry

Cyclic voltammetry data was recorded using a 760c potentiostat (CHInstruments, Austin, Tex.) with iR correction. Tests were conductedusing glassy carbon working electrodes (Bioanalytical Systems, Inc.,West Lafayette, Ind.), Ag/AgCl reference electrodes (BioanalyticalSystems, Inc. West Lafayette, Ind.) and platinum wire counter electrodes(Alfa Aesar, Ward Hill, Mass.). Working electrodes were polishedaccording to the supplier's instructions before each experiment.Reference electrodes were calibrated against a “master” Ag/AgClelectrode known to have a potential of +0.210 V vs. NHE as known bythose skilled in the art of electrochemistry. Solutions were spargedwith argon for at least 5 minutes before each experiment. Allexperiments were performed at ambient temperatures (17-22° C.). Nosupporting electrolytes were added unless otherwise specified. All datawere collected at a scan rate of 100 mV/s unless otherwise specified.Under these conditions, hydrogen evolution became significant atpotentials more negative than −0.80 V vs. RHE and oxygen evolutionbecame significant at potentials more positive than +2.20 V vs. RHE.

Example 1.3 Experimental Procedure for a 5 cm² Active Area Flow Battery

Cell hardware designed for 5 cm² active area and modified for acid flowwas obtained from Fuel Cell Technologies (Albuquerque, N. Mex.). Carbonfelt, nominally 3 mm thick, was obtained from Alfa Aesar (Ward Hill,Mass.) and MGL 370 carbon paper was obtained from Fuel Cell Earth(Stoneham, Mass.). Felts were dip-coated with a suspension of VulcanXC-72 carbon (Cabot Corp., Boston, Mass.) and NAFION™ (Ion-Power, NewCastle, Del.) and air-dried before use and carbon papers were used asreceived. NAFION™ HP, XL, or NR-212 cation exchange membranes wereobtained from Ion-Power in the H+ form and were used as received. Viton™gaskets were obtained from McMaster Carr (Robinsville, N.J.) and werecut to allow for a 5 cm² active area with ˜1 cm² areas left above andbelow the felts for electrolyte ingress and egress from the positive andnegative compartments of the cell. The cell was assembled using gasketsthat provided a compression of ˜25% of the measured thickness of thefelts or papers. The membranes and electrodes were not pretreated beforeassembly. The electrolyte reservoirs were fashioned from Schedule 80 PVCpiping with PVDF tubing and compression fittings. Masterflex™ L/Speristaltic pumps (Cole Parmer, Vernon Hills, Ill.) were used withTygon™ tubing. Electrolytes were sparged with UHP argon through anoil-filled bubbler outlet before electrochemical testing and a headpressure of argon was maintained during the testing. An ArbinInstruments BT2000 (College Station, Tex.) was used to test theelectrochemical performance, and a Hioki 3561 Battery HiTESTER(Cranbury, N.J.) was used to measure the AC resistance across the cell.

In a typical experiment, 50 mL each of electrolyte containing activematerial for the positive and negative electrode were loaded intoseparate reservoirs and sparged with argon for 20 minutes whilecirculating the electrolytes through the cell. The electrolytes werecharged to 40% SOC (calculated from the concentrations of the activematerials and the volumes of the electrolyte), the iV response of thecell was obtained, and then the electrolytes were cycled between 40 and60% SOC. An analog output from the Hioki battery tester was recorded tomonitor changes in the membrane and contact resistances.

Example 2

A redox flow battery cell was assembled according to the methodsdescribed in Example 1.3 using titanium tris-catecholate (Ti^(4+/3+)(cat)₃ ^(2−/3−)) and ferri/ferro-cyanide (Fe^(3+/2+) (CN)₆ ^(3−/4−))metal ligand coordination compounds as active materials for the negativeand positive electrolytes, respectively. The active materials wereprepared at concentrations of 0.5 M in 0.5 M pH 11 Na₂SO₄ supportingelectrolyte (negative electrolye, or negolyte) or no supportingelectrolyte (positive electrolyte, or posolyte) and were flowed at 100mL/min through the flow battery cell assembled using 5 cm² carbon feltelectrodes and a NAFION™ cation selective membrane (50 μm thick) in Naform. The cell was initially charged from 0 to 50% state of chargebefore several charge/discharge cycles was collected by sweeping thecell current from open circuit to ˜150 mA/cm² and monitoring theresulting cell potential, FIG. 2. At open circuit, a cell potential of1.63 V was observed as expected for equilibrium cell potential at 50%SOC based on the externally measured E_(1/2) values for Ti^(4+/3+)(cat)₃ ^(2−/3−) and Fe^(3+/2+) (CN)₆ ^(3−/4−). Charge/discharge cyclingrevealed well behaved, reproducible voltage/current vs. time traces,demonstrating promising durability, FIG. 2. An RT voltage efficiency of69% was measured for this system at 150 mA/cm². Typical resistancesmeasured by the Hioki Battery Tester for the membrane and contactresistance component of cells built with NR212, XL, and HP membraneswere 0.77, 0.60, and 0.5 ohm-cm², respectively.

FIG. 3 displays the charge/discharge characteristics for a flow batteryof the present invention wherein the negative and positive activematerials comprise Ti^(4+/3+) (cat)₃ ^(2−/3−) and Fe^(3+/2+) (CN)₆^(3−/4−), respectively. The cell potential increases as the battery ischarged and decreases as the battery is discharged.

Example 3

A redox flow battery cell was assembled according to the methodsdescribed in Example 1.3 using titanium tris-catecholate (Ti^(4+/3+)(cat)₃ ²⁻³⁻) and ferri/ferro-cyanide (Fe^(3+/2+) (CN)₆ ^(3−/4−)) metalligand coordination compounds as active materials for the negative andpositive electrolytes, respectively. In a typical cell, stable voltageswere observed upon repeatedly charging to 60% SOC and discharging to 40%SOC (see FIG. 4) when the discharge energy for each cycle was 99.8% ofthe charge energy, indicative of 99.8% roundtrip current efficiency.This was achieved by using a constant current density (e.g., 150 mA/cm²)for both charge and discharge but with a discharge time that wasslightly shorter than (i.e., 99.8% of) the charge time. Under theseconditions, the open circuit voltages at 40 and 60% SOC were stable forextended periods of time.

Crossover flux data were obtained by measuring the concentrations of Feand Ti in each electrolyte at the beginning and end of a suitablylengthy battery test, typically one to two weeks in duration for amembrane area of 7 cm². The concentrations were determined byInductively Coupled Plasma—Mass Spectrometry (ICP-MS) experimentsperformed by Evans Analytical Group, Syracuse, N.Y. The moles of Fe inthe Ti-containing electrolyte before the test were subtracted from thenumber of moles in the same electrolyte at the end of the test. This wasconverted to a flux by dividing the moles by the membrane area and thetest duration.

Typical fluxes for boiled DuPont Nafion™ NR212 (50 micron thick) were5.0×10⁻⁸ mol cm⁻² day⁻¹ for ferri/ferrocyanide and 6.5×10⁻⁸ mol cm⁻²day⁻¹ for titanium triscatecholate. For unboiled DuPont Naflon™ HP (20micron thick), the measured fluxes were 1.1×10⁻⁵ and 3.3×10⁻⁶ mol cm⁻²day⁻¹ for the above iron and titanium complexes, respectively. It shouldbe noted that these fluxes are substantially lower than 1% of the totalcurrent (and thus the total moles of ions passed across the membrane)during this time. For example, in the NR212 test above, 6.4×10⁻² mol oftotal ions were passed over 6.8 days of operation at 100 mA/cm²,approximately 6 orders of magnitude larger than the amount of activematerial ion crossover. These results are believed to berepresentative/typical for the compounds described herein.

Example 4

A redox flow battery cell was assembled according to the general methodsdescribed in Example 1.3, again using titanium bis-catecholatemono-pyrogallate (Ti^(4+/3+) (cat)₂(gal)^(2−/3−)) andferri/ferro-cyanide (Fe^(3+/2+) (CN)₆ ^(3−/4−)) metal ligandcoordination compounds as active materials for the negative and positiveelectrolytes, respectively. In this example the carbon felt electrodeswere replaced with TORAY carbon paper electrodes that were catalyzedwith Vulcan carbon and NAFION™ in a manner similar to that of Example 2.Additionally, flow fields of the “interdigitated” type were employed.The active material solution concentrations were increased to 1.5 M andthe cell performance was evaluated by monitoring the cell potential onboth charge and discharge cycles as a function of current density. Ascan be seen in FIG. 5, the cell maintains round trip voltageefficiencies of 84%, 79%, and 73% at current densities of 150, 200, and250 mA/cm², respectively. In this configuration the flow battery activematerials exhibited an energy density of 32.79 Wh/L.

The results of analogous experiments using Ti^(4+/3+) (cat)₃ ^(2−/3−)and Fe^(3+/2+) (CN)₆ ^(3−/4−) are shown in FIG. 6 and FIG. 7.

Example 5

A redox flow battery cell was assembled according to the methodsdescribed in Example 1.3 using titanium bis-lactate mono-salicylate([Ti^(4+/3+) (lactate)₂(salicylate)]^(2−/3−)) and ferri/ferro-cyanide([Fe^(3+/2+) (CN)₆]^(3−/4−)) metal ligand coordination compounds asactive materials for the negative and positive electrolytes,respectively. The active material solutions were prepared atconcentrations of 1 M with no additional supporting electrolyte and wereflowed at 100 mL/min through the flow battery cell assembled using 5 cm²carbon paper electrodes and a NAFION™ cation selective membrane (25 μmthick) in Na form. The cell was initially charged from 0 to 25% state ofcharge before charge/discharge cycles were collected by charging anddischarging the cell at 150 or 100 mA/cm² and monitoring the resultingcell potential, FIG. 21 (where visually wider cycles were taken at 100instead of 150 mA/cm²). At open circuit, a cell potential of 1.60 V wasobserved as expected for equilibrium cell potential at 50% SOC based onthe externally measured E_(1/2) values for [Ti^(4+/3+)(lactate)₂(salicylate)]^(2−/3−) and [Fe^(3+/2+) (CN)₆]^(3−/4−).Charge/discharge cycling revealed well behaved, reproduciblevoltage/current vs. time traces, demonstrating promising durability,FIG. 21. An encouraging RT voltage efficiency of 67% was measured forthis system at 150 mA/cm². Typical resistances measured by the HiokiBattery Tester for the membrane and contact resistance component ofcells built with NR212, XL, and HP membranes were 0.77, 0.60, and 0.5ohm-cm², respectively.

Example 6

A redox flow battery cell was assembled according to the methodsdescribed in Example 1.3 using titanium bis-lactate mono-glycolic acid([Ti^(4+/3+) (lactate)₂α-hydroxyacetate)]^(2−/3−)) andferri/ferro-cyanide ([Fe^(3+/2+) (CN)₆]^(3−/4−)) metal ligandcoordination compounds as active materials for the negative and positiveelectrolytes, respectively. In a typical cell, stable voltages wereobserved upon repeatedly charging to 75% SOC and discharging to 25% SOC(see FIG. 22) when the discharge energy for each cycle was 99.8% of thecharge energy, indicative of 99.8% roundtrip current efficiency. Thiswas achieved by using a constant current density (e.g., 150 mA/cm²) forboth charge and discharge but with a discharge time that was slightlyshorter than (i.e., 99.8% of) the charge time. Under these conditions,the open circuit voltages at 25 and 75% SOC were stable for extendedperiods of time.

Example 7 Cyclic Voltammetry Data

The following experiments provide information as to the nature of thehalf-cell performance for the indicated materials. As described above,certain embodiments of the present invention include those flowbatteries comprising these, or analogous, materials which would providefull cell performance reflective of the reported half-cell performance,and such embodiments are considered within the scope of the presentinvention.

Table 2A Exemplary electrochemical couples described herein SolubilityCharge E_(1/2), V vs. (Molar), Density Couple RHE pH FIG. 25° C. (Ah/L)Al(citrate)₂(catecholate)^(2−/3−) 1.25 11.5 8 0.5 13.4 Fe(catecholate)₃^(2−/3−) −0.50 11 10 1.5 40.2 Ti(catecholate)₃ ^(2−/3−) −0.45 11 15 1.026.8 Ti(pyrogallate)₃ ^(2−/3−) −0.55 9.8 9 1.6 42.9Ti(catecholate)₂(pyrogallate)^(2−/3−) −0.50 11 11 1.5 40.2Ti(catecholate)₂(ascorbate)^(2−/3−) −0.55 10 14 1.5 40.2Ti(catecholate)₂(gluconate)^(2−/3−) −0.60 9 13 1.5 40.2Ti(catecholate)₂(lactate)^(2−/3−) −0.49 9 12 1.5 40.2Ti(catecholate)(pyrogallate)(lactate)^(2−/3−) −0.70 8.5 16 1.5 40.2Ti(citrate)₃ −0.04 5 17 2.0 53.6 Fe(CN)₆ ^(3−/4−) 1.18 11 18 1.5 40.2Cr(CN)₆ ^(3−/4−) −0.60 9 19 1.5 40.2 Mn(CN)₆ ^(3−/4−) −0.60 9 20 1.540.2 Table 2B Exemplary electrochemical couples described hereinSolubility Charge E_(1/2), V vs. (Molar), Density Couple RHE pH FIG.*25° C. (Ah/L) Ti^(IV/III)(lactate)₁ −0.34 3.6 N/S 1.75 46.9Ti^(IV/III)(lactate)₁ −0.40 5.6 25 1.75 46.9 Ti^(IV/III)(lactate)₁ −0.549 26 1.75 46.9 Ti^(IV/III)(lactate)₂ −0.03 2 27 1.75 46.9Ti^(IV/III)(lactate)₂ −0.40 3.6 28 1.75 46.9 Ti^(IV/III)(lactate)₂ −0.409 29 1.75 46.9 Ti^(IV/III)(lactate)₁(malate)₂ −0.40 9.9 30 1.5 40.2Ti^(IV/III)(malate)₂(salicylate) −0.48 10 31 1.5 40.2Ti^(IV/III)(lactate)₂(glycinate) −0.50 9.9 32 1.5 40.2Ti^(IV/III)(lactate)₂(salicylate) −0.48 10 33 1.5 40.2Ti^(IV/III)(salicylate)₂(lactate) −0.50 9.8 34 1.5 40.2Ti^(IV/III)(α-hydroxyacetate)₂(salicylate) −0.48 10 35 1.5 40.2Ti^(IV/III)(malate)₂(salicylate) −0.50 10 N/S 1.5 40.2Ti^(IV/III)(α-hydroxyacetate)₂(lactate) −0.50 10 36 1.5 40.2Ti^(IV/III)(lactate)₂(α-hydroxyacetate) −0.50 10 N/S 1.5 40.2Ti^(IV/III)(lactate)₃ −0.45 10 N/S 1.75 46.9 Ti^(IV/III)(salicylate)₃−0.25 8.6 23 0.5 13.4 Fe^(III/II)(salicylate)₃ −0.10 9.3 24 0.5 13.4Fe^(III/II)(malate)₃ −0.30 9.2 37 1.0 26.8Fe^(III/II)(α-hydroxyacetate)₃ −0.50 8.1 38 1.0 26.8Fe^(III/II)(lactate)₂(salicylate)₁ −0.39 8.7 N/S 1.0 26.8Fe^(III/II)(lactate)₂(glycinate)₁ +0.30 6.7 N/S 1.0 26.8Fe^(III/II)(lactate)₂ +0.45 2.6 40 1.5 40.2 Fe^(III/II)(lactate)₁ +0.113.1 39 1.5 40.2 Fe(CN)₆ ^(3−/4−) +1.18 11 18 1.5 40.2Al(citrate)₂(catecholate)^(2−/3−) +1.25 11.5 8 0.5 13.4Fe^(III/II)(H₂O)₆ +0.77 0 N/S 2 53.6 Ce^(IV/III)(H₂O)_(x) +1.75 0 N/S0.5 13.4 *N/S = Not Shown

Table 3A Calculated OCVs and theoretical energy density (Wh/L) forvarious other electrolyte couple pairs calculated from data in Table 2.Fe(CN)₆ ^(3−/4−) Al(cit)₂(cat)^(2−/3−) Energy Energy OCV Density OCVDensity Couple (V) (Wh/L) (V) (Wh/L) Mn(CN)₆ ^(3−/4−) 1.78 35.8 1.8512.4 Fe(catecholate)₃ ^(2−/3−) 1.68 33.8 1.75 11.7 Ti(catecholate)₃^(2−/3−) 1.63 21.8 1.70 11.4 Ti(pyrogallate)₃ ^(2−/3−) 1.73 34.8 1.8012.1 Ti(catecholate)₂(pyrogallate)^(2−/3−) 1.68 33.8 1.75 11.7Ti(catecholate)₂(ascorbate)^(2−/3−) 1.73 34.8 1.80 12.1Ti(catecholate)₂(gluconate)^(2−/3−) 1.78 35.8 1.85 12.4Ti(catecholate)₂(lactate)^(2−/3−) 1.67 33.6 1.74 11.7Ti(catecholate)(pyrogallate)- 1.73 34.8 1.80 12.1 (lactate)^(2−/3−)Ti(citrate)₃ 1.22 24.5 1.29 8.6 Table 3B Calculated OCVs and theoreticalenergy density (Wh/L) for various electrolyte couple pairs calculatedfrom data in Table 2. Fe(CN)₆ ^(3−/4−) Al(cit)₂(cat)^(2−/3−) EnergyEnergy OCV Density OCV Density Couple (V) (Wh/L) (V) (Wh/L)Ti^(IV/III)(lactate)₁ 1.60 34.9 1.67 25.2 Ti^(IV/III)(lactate)₂ 1.4631.8 1.53 23.1 Ti^(IV/III)(lactate)₃ 1.57 34.2 1.64 24.7Ti^(IV/III)(salicylate)₃ 1.29 17.3 1.36 9.1Ti^(IV/III)(lactate)₁(malate)₂ 1.51 30.4 1.58 21.2Ti^(IV/III)(malate)₂(salicylate) 1.60 32.2 1.67 22.4Ti^(IV/III)(lactate)₂(glycinate) 1.61 32.4 1.68 22.5Ti^(IV/III)(lactate)₂(salicylate) 1.60 32.2 1.67 22.4Ti^(IV/III)(salicylate)₂(lactate) 1.61 32.3 1.68 22.5 Ti^(IV/III)(α-1.60 32.2 1.67 22.4 hydroxyacetate)₂(salicylate)Ti^(IV/III)(malate)₂(sal) 1.62 32.6 1.69 22.6Ti^(IV/III)(α-hydroxyacetate)₂(lactate) 1.62 32.6 1.69 22.6Ti^(IV/III)(lactate)₂(α-hydroxyacetate) 1.62 32.6 1.69 22.6Fe^(III/II)(salicylate)₃ 1.18 15.8 1.25 8.4 Fe^(III/II)(malate)₃ 1.3723.0 1.44 14.5 Fe^(III/II)(α-hydroxyacetate)₃ 1.51 25.3 1.58 15.9

TABLE 4 Calculated OCVs and theoretical energy density (Wh/L) forvarious electrolyte couple pairs calculated from data in Table 2 inmildly acidic solutions. 2M Fe^(III/II), 0.5M Ce^(IV/III), pH 2 pH 2Energy Energy OCV Density OCV Density Couple (V) (Wh/L) (V) (Wh/L)Ti^(IV/III)(lactate)₁ 1.32 33.2 2.30 34.7 Ti^(IV/III)(lactate)₂ 0.9223.1 1.90 28.6

Example 7.1

Using an Al(citrate)₂(catecholate)^(2−/3−) couple (E_(1/2)=˜1.25 V vs.RHE) as a demonstrative case, a high potential was observed withwell-behaved electrochemical signatures at glassy carbon electrodes,FIG. 8. When coupled with the Ti⁴⁺(catecholate)₃ ²⁻ complex describedabove these pairs may give aqueous battery pairs with OCVs of ˜1.7-1.9V. When coupled with the Ti and Fe complexes comprising α- and β-hydroxyacid ligands, these pairs may give aqueous battery pairs with OCVs of˜1.3-1.6 V.

Examples 7.2 through 7.5

FIG. 9 (for titanium tris-pyrogallate) and FIG. 10 (for irontris-catecholate) illustrate the CV curves resulting from the use ofcatecholate-like ligands over a range of low and negative operatingpotentials, under conditions described above, showing the goodelectrochemical reversibility of these systems under these conditions.FIG. 23 (for titanium tris-salicylate) and FIG. 24 (for irontris-salicylate) illustrate the CV curves resulting from the use ofβ-hydroxy acid ligands over a range of low and negative operatingpotentials, under conditions described above, showing the goodelectrochemical reversibility of these systems under these conditions.

Examples 7.6 through 7.12

FIG. 11 (NaK[Ti(catecholate)₂(pyrogallate)]), FIG. 12(NaK[Ti(catecholate)₂(lactate)]), FIG. 13(NaK[Ti(catecholate)₂(gluconate)]), FIG. 14(NaK[Ti(catecholate)₂(ascorbate)]), FIG. 15 (Na₂-[Ti(catecholate)₃]),FIG. 16 (NaK[Ti(catecholate)(pyrogallate)(lactate)]), and FIG. 17(Na₄-[Ti(citrate)₃]) illustrate the CV curves resulting from the use ofseveral mixed ligand or tris-citrate systems over a range of low andnegative operating potentials, under conditions described above, showingthe good electrochemical reversibility of these systems under theseconditions.

Examples 7.13 through 7.29

FIG. 25 through FIG. 41 illustrate the CV curves resulting from the useof several mixed ligand or tris-α-hydroxy acid systems over a range oflow and negative operating potentials, under conditions described above,showing the good electrochemical reversibility of these systems underthese conditions.

Example 7.30 Ferrocyanide Samples

Solid Na₄Fe(CN)₆.10H₂O (33.89 g, 0.070 mol) and K₄Fe(CN)₆.3H₂O (29.57 g,0.070 mol) were stirred in 80 mL deionized water. To dissolve thesolids, sufficient water was then slowly added to provide a samplecontaining ca. 1.5 M of Fe(CN)₆ ⁴⁻. This solubility was unexpected giventhat the solubilities of Na₄Fe(CN)₆.10H₂O and K₄Fe(CN)₆.3H₂O are eachknown in the art to be less than 0.7 M at the same ambient temperatures.

The 1.5 M [Fe(CN)₆]⁴⁻ solution was interrogated by cyclic voltammetry,using a glassy carbon working electrode. FIG. 18. In these experiments,sufficient solid sodium potassium hydrogen phosphate, NaOH, and KOH wasadded to the 1.4 M [Fe(CN)₆]⁴⁻ solution to yield a working solutionhaving a pH of 11.1 (ratio N⁺/K⁺˜1) and containing 1.4 M [Fe(CN)₆]⁴⁻ and0.1 M phosphate.

Examples 7.31 and 7.32

FIG. 19 (K₃-[Cr(CN)₆]) and FIG. 20 (K₃[Mn(CN)₆]) illustrate the CVcurves resulting from the use of two other hexacyanide systems over arange of low and negative operating potentials, under conditionsdescribed above, showing the good electrochemical reversibility of thesesystems under these conditions.

Many of the embodiments thus far have been described in terms of flowbatteries in which at least one metal ligand coordination compounds isdescribed by the formula M(L1)_(X)(L2)_(y)(L3)_(z) ^(m). It should beappreciated, however, that other embodiments include those where thehexacyanide compounds described herein may provide the basis of both ofthe positive and negative electrolytes. From FIG. 20, for example, itshould be apparent that the [Mn(CN)₆]^(3−/4−) and [Mn(CN)₆]^(4−/5−)couples, in addition to providing the basis of either positive ornegative electrolytes, in combination with other complementaryelectrolytes described herein as M(L1)_(x)(L2)_(y)(L3)_(z) ^(m), mayalso provide the basis for both the positive and negative electrolytesin a flow battery system. Similarly, independent embodiments alsoinclude those where the positive electrolyte comprises [Fe(CN)₆]^(3−/4−)and the negative electrolyte comprises [Cr(CN)₆]^(3−/4−) or[Mn(CN)₆]^(3−/4−).

As those skilled in the art will appreciate, numerous modifications andvariations of the present invention are possible in light of theseteachings, and all such are contemplated hereby. For example, inaddition to the embodiments described herein, the present inventioncontemplates and claims those inventions resulting from the combinationof features of the invention cited herein and those of the cited priorart references which complement the features of the present invention.Similarly, it will be appreciated that any described material, feature,or article may be used in combination with any other material, feature,or article, and such combinations are considered within the scope ofthis invention.

The disclosures of each patent, patent application, and publicationcited or described in this document are hereby incorporated herein byreference, each in its entirety.

What is claimed:
 1. A flow battery comprising: a first aqueouselectrolyte comprising a first redox active material; a second aqueouselectrolyte comprising a second redox active material; a first electrodein contact with said first aqueous electrolyte; a second electrode incontact with said second aqueous electrolyte; and a separator disposedbetween said first aqueous electrolyte and said second aqueouselectrolyte; wherein: (a) each of the first and second redox activematerials comprises a metal-ligand coordination compound thatindependently exhibits substantially reversible electrochemicalkinetics; or (b) the first, second, or both redox active materialscomprise a metal ligand coordination compound in concentrations of atleast 0.75 M; or (c) the first, second, or both first and second redoxactive materials comprise a metal ligand coordination compound and saidflow battery is capable of operating with a current density of at least100 mA/cm² and a round trip voltage efficiency or at least 70%; or (d)the first, second, or both redox active materials comprise a metalligand coordination compound and the separator has a thickness of 100micron or less; or (e) the first, second, or both redox active materialscomprise a metal ligand coordination compound and wherein the energydensity of the electrolytes is at least 30 Wh/L; or (f) the flow batterycomprises any combination of (a) through (e).
 2. A flow batterycomprising: a first aqueous electrolyte comprising a first redox activematerial; a second aqueous electrolyte comprising a second redox activematerial; a first electrode in contact with said first aqueouselectrolyte; a second electrode in contact with said second aqueouselectrolyte and a separator disposed between said first aqueouselectrolyte and said second aqueous electrolyte; wherein the first orsecond redox active material, or both the first and second redox activematerials comprise a metal ligand coordination compound having a formulacomprising M(L1)_(x)(L2)_(y)(L3)_(z) ^(m), where M is independently anon-zero valent metal or metalloid of Groups 2-16, including lanthanidesand actinides, wherein x, y, and z are independently 0, 1, 2, or 3, and1≦x+y+z≦3; m is independently −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, or 5;and L1, L2, and L3 are each independently ascorbate, citrate, aglycolate, gluconate, glycinate, α-hydroxyalkanoate, β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, a polyol, sarcosinate,salicylate, lactate, or a compound having structure according to FormulaI, or an oxidized or reduced form thereof:

wherein Ar is a 5-20 membered aromatic moiety, optionally comprising oneof more O, N, or S heteroatoms; X₁ and X₂ are independently —OH, —NHR′,—SH, or an anion thereof, X₁ and X₂ being positioned ortho to oneanother; R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆alkyl, C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, aboric acid or a salt thereof, carboxy acid or a salt thereof, C₂₋₆carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol; R′ is independently H or C₁₋₃ alkyl; and n is 0, 1, 2, 3, 4,5, 6, 7, 8, 9, or
 10. 3. The flow battery of claim 2, wherein the firstand second redox active materials comprise a metal ligand coordinationcompound.
 4. The flow battery of claim 2, wherein (a) x=3, y=z=0; (b)x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0; or(f) x=1, y=z=0.
 5. The flow battery of claim 2, wherein the firstaqueous electrolyte comprises a first metal-ligand coordination compoundand the second aqueous electrolyte comprises a second metal-ligandcoordination compound, wherein the first and second metal-ligandcoordination compounds are different.
 6. The flow battery of claim 2,wherein the first, the second, or both the first and second metal-ligandcoordination compound comprises at least one ligand having a structureaccording to Formula I.
 7. The flow battery of claim 1 or 2, wherein thefirst, the second, or both of the redox-active metal ligand coordinationcompounds comprises at least one ligand having a structure according toFormula IA, IB, or IC:

wherein X₁ and X₂ are independently —OH, —NHR′, —SH, or an anionthereof; R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆alkyl, a boric acid or a salt thereof, carboxy acid or a salt thereof,C₂₋₆ carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acidor a salt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol; R′ is independently H or C₁₋₃ alkyl; and n is 0-4.
 8. Theflow battery of claim 7, wherein X₁ and X₂ are both OH or an anionthereof; R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid ora salt thereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano,halo, hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol; and nis
 1. 9. The flow battery of claim 1 or 2, where the first metal-ligandcoordination compound comprises at least one ascorbate, a catecholate,citrate, a glycolate or polyol, gluconate, glycinate,α-hydroxyalkanoate, β-hydroxyalkanoate, γ-hydroxyalkanoate, malate,maleate, phthalate, sarcosinate, salicylate, or lactate ligand.
 10. Theflow battery of claim 1 or 2, where the second metal-ligand coordinationcompound comprises at least one ascorbate, a catecholate, citrate, aglycolate or polyol, gluconate, glycinate, α-hydroxyalkanoate,β-hydroxyalkanoate, γ-hydroxyalkanoate, malate, maleate, phthalate,sarcosinate, salicylate, or lactate ligand.
 11. The flow battery ofclaim 1 or 2, where the first metal-ligand coordination compoundcomprises at least one ligand of Formula I, IA, IB, or IC.
 12. The flowbattery of claim 1 or 2, where the second metal-ligand coordinationcompound comprises at least one ligand of Formula I, IA, IB, or IC. 13.The flow battery of claim 1 or 2, wherein either the first or the secondor both the first and second metal-ligand coordination compoundcomprises Al, Ca, Ce, Co, Cr, Fe, Mg, Mn, Mo, Si, Sn, Ti, W, Zn, or Zr.14. The flow battery of claim 1 or 2, wherein the first metal-ligandcoordination compound comprises Al³⁺, Ca²⁺, Ce⁴⁺, Co³⁺, Cr³⁺, Fe³⁺,Mg²⁺, Mn³⁺, Mo⁶⁺, Si⁴⁺, Sn⁴⁺, Ti⁴⁺, W⁶⁺, Zn²⁺, or Zr⁴⁺.
 15. The flowbattery of claim 1 or 2, wherein the second metal-ligand coordinationcompound comprises Al³⁺, Ca²⁺, Ce⁴⁺, Co³⁺, Cr³⁺, Fe³⁺, Mg²⁺, Mn³⁺, Mo⁶⁺,Si⁴⁺, Sn⁴⁺, Ti⁴⁺, W⁶⁺, Zn²⁺, or Zr⁴⁺.
 16. The flow battery of claim 1 or2, wherein the first metal-ligand coordination compound comprises Cr,Ti, or Fe.
 17. The flow battery of claim 1 or 2, wherein the secondmetal-ligand coordination compound comprises Cr, Ti, or Fe.
 18. The flowbattery of claim 1 or 2, wherein the second metal-ligand coordinationcompound comprises an iron hexacyanide compound.
 19. The flow battery ofclaim 1 or 2, wherein either the first or the second or both the firstand second metal-ligand coordination compounds is characterized ashaving a hydrodynamic diameter and the separator is characterized ashaving a mean pore size, wherein the hydrodynamic diameter of thecoordination compound is larger than the mean pore size of theseparator.
 20. The flow battery of claim 1 or 2, wherein either or bothof the first or the second metal-ligand coordination compound arepresent in the first or second electrolyte, respectively, at aconcentration of at least 0.75 M.
 21. The flow battery of claim 2,wherein the first and second metal-ligand coordination compounds eachexhibits substantially reversible electrochemical kinetics.
 22. The flowbattery of claim 2, wherein the cell exhibits a round trip voltageefficiency of at least 70%, when measured at 200 mA/cm².
 23. The flowbattery of claim 2, wherein the flow battery is capable of operating, oris operating, with a current density of at least 100 mA/cm² and a roundtrip voltage efficiency of at least 70%.
 24. The flow battery of claim2, wherein the separator has a thickness of about 100 micron or less.25. The flow battery of claim 2, wherein the energy density of theelectrolytes is at least about 30 Wh/L.
 26. The flow battery of claim 2,wherein the cell retains at least 70% round trip voltage efficiency whensubjected to 10 charge/discharge cycles.
 27. The flow battery of claim1, wherein the first, second, or first and second redox active materialcomprises a metal ligand coordination complex having a formulacomprising M(L1)_(X)(L2)_(y)(L3)_(z) ^(m), where x, y, and z areindependently 0, 1, 2, or 3, and 1≦x+y+z≦3; and where M is Al, Ca, Ce,Co, Cr, Fe, Mg, Mn, Mo, S, Sn, Ti, W, Zn, or Zr; L1, L2, and L3 are eachindependently ascorbate, a catecholate, citrate, a glycolate or polyol,gluconate, glycinate, α-hydroxyalkanoate, β-hydroxyalkanoate,γ-hydroxyalkanoate, malate, maleate, phthalate, sarcosinate, salicylate,lactate or a compound having structure according to Formula I, or anoxidized or reduced form thereof:

wherein Ar is a 5-20 membered aromatic moiety, optionally comprising oneof more O, N, or S heteroatoms; X₁ and X₂ are independently —OH, —NHR′,—SH, or an anion thereof, X₁ and X₂ being positioned ortho to oneanother; R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆alkyl, C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, aboric acid or a salt thereof, carboxy acid or a salt thereof, C₂₋₆carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol; R′ is independently H or C₁₋₃ alkyl; and m is +1, 0, −1, −2,−3, −4, or −5.
 28. The flow battery of claim 27, where (a) x=3, y=z=0;(b) x=2, y=1, z=0; (c) x=1, y=1, z=1; (d) x=2, y=1, z=0; (e) x=2, y=z=0;or (f) x=1, y=z=0.
 29. The flow battery of claim 1, wherein either thefirst or the second or both the first and second metal-ligandcoordination compound comprises at least one ligand having a structureaccording to Formula I.
 30. The flow battery of claim 28, wherein eitheror both of the redox-active metal ligand coordination compoundscomprises at least one ligand having a structure according to FormulaIA, IB, or IC:

wherein X₁ and X₂ are independently —OH, —NHR′, —SH, or an anionthereof; R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆alkyl, a boric acid or a salt thereof, a carboxy acid or a salt thereof,C₂₋₆ carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, a sulfonicacid or a salt thereof, phosphonate, phosphonic acid or a salt thereof,or a polyglycol; R′ is independently H or C₁₋₃ alkyl; and n is 0-4. 31.The flow battery of claim 30, wherein X₁ and X₂ are both OH or an anionthereof; R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid ora salt thereof, carboxy acid or a salt thereof, C₂₋₆ carboxylate, cyano,halo, hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol; and nis
 1. 32. The flow battery of claim 1 or 2, where the secondmetal-ligand coordination compound comprises at least one catechol orpyrogallol ligand.
 33. The flow battery of any claim 1 or 2, wherein thesecond metal-ligand coordination compound is an iron hexacyanidecompound.
 34. The flow battery of claim 1 or 2, wherein either the firstor the second or both the first and second metal-ligand coordinationcompounds is characterized as having a hydrodynamic diameter and theseparator is characterized as having a mean pore size, wherein thehydrodynamic diameter of the coordination compound is larger than themean pore size of the separator.
 35. A system comprising a flow batteryof claim 1 or 2, and further comprising: (a) a first chamber containingthe first aqueous electrolyte and a second chamber containing the secondaqueous electrolyte; (b) at least one electrolyte circulation loop influidic communication each electrolyte chamber, said at least oneelectrolyte circulation loop comprising storage tanks and piping forcontaining and transporting the electrolytes; (c) control hardware andsoftware; and (d) an optional power conditioning unit.
 36. The system ofclaim 35, the system connected to an electrical grid configured toprovide renewables integration, peak load shifting, grid firming,baseload power generation/consumption, energy arbitrage, transmissionand distribution asset deferral, weak grid support, frequencyregulation, or a combination thereof.
 37. The system of claim 35, thesystem configured to provide stable power for remote camps, forwardoperating bases, off-grid telecommunications, or remote sensors.
 38. Amethod of operating a flow battery of claim 1 or 2, said methodcomprising charging said battery by the input of electrical energy ordischarging said battery by the removal of electrical energy.
 39. Amethod of charging a flow battery or system of claim 1 or 2, with anassociated flow of electrons, said method comprising applying apotential difference across the first and second electrode, so as to:(a) reduce the first redox active metal-ligand coordination compound; or(b) oxidize the second redox active metal-ligand coordination compound;or (c) both (a) and (b).
 40. A method of discharging the flow battery orsystem of claim 1 or 2, with an associated flow of electrons, saidmethod comprising applying a potential difference across the first andsecond electrode so as to: (a) oxidize the first redox activemetal-ligand coordination compound; or (b) reduce the second redoxactive metal-ligand coordination compound; or (c) both (a) and (b).